When the weather turns chilly, there’s nothing more comforting than a bowl of Mushroom & Caramelized Onion Soup — especially when it’s topped with gooey, golden cheesy toasties.
This recipe takes inspiration from the French classic but adds a hearty, earthy twist with sautéed mushrooms. Slowly caramelized onions bring sweetness and depth, while mushrooms add umami richness, all simmered together in a savory beef broth infused with thyme and bay leaf.
Finished with toasted bread smothered in melted cheese, it’s the kind of rustic, feel-good meal that turns an ordinary evening into something special.
Why You’ll Love This Mushroom & Caramelized Onion Soup
- Deep, complex flavor — sweet onions meet earthy mushrooms in a rich broth.
- Cheesy toasties — crispy on the outside, gooey in the center.
- Restaurant-quality, homemade feel — no fancy equipment required.
- Perfect for cozy dinners — comforting, hearty, and full of aroma.
- Customizable — swap cheeses, add herbs, or make it vegetarian.
Why You Should Try This Recipe
This soup captures everything comforting about homemade cooking — it’s simple but layered with flavor. Caramelizing onions slowly brings out their natural sweetness, while mushrooms lend that deep, meaty savoriness that makes every spoonful rich and satisfying.
The cheesy toasties take it over the top — crusty bread topped with melted Gruyère or mozzarella that soaks up the broth like a sponge. It’s cozy, elegant, and completely irresistible.
Ingredients You’ll Need
For the Soup:
- 4 tablespoons unsalted butter
- 4 large yellow onions, thinly sliced
- 2 cloves garlic, finely chopped
- 2 bay leaves
- 2 sprigs fresh thyme (or ½ teaspoon dried thyme)
- ¼ cup all-purpose flour
- 2 quarts (8 cups) beef broth
- 8 oz mushrooms, sliced (cremini, button, or mix)
- Salt & black pepper, to taste
- Optional: splash of dry white wine or sherry for deglazing
For the Cheesy Toasties:
- 1 small baguette or crusty bread, sliced
- 1 ½ cups shredded Gruyère, mozzarella, or Swiss cheese
- 1 tablespoon olive oil or butter, melted
- Optional garnish: fresh parsley or thyme
How to Make Mushroom & Caramelized Onion Soup with Cheesy Toasties (Step by Step)
Step 1: Caramelize the onions
In a large pot or Dutch oven, melt butter over medium heat. Add sliced onions and a pinch of salt. Cook, stirring frequently, for 25–30 minutes until onions turn golden brown and caramelized. If they start to stick, add a splash of water or wine to deglaze the pan.
Step 2: Add garlic and herbs
Stir in chopped garlic, bay leaves, and thyme. Cook for 1–2 minutes until fragrant.
Step 3: Add mushrooms
Add the sliced mushrooms to the pot and cook for 8–10 minutes, stirring occasionally, until tender and lightly browned.
Step 4: Add flour
Sprinkle flour over the mixture and stir for 1–2 minutes to create a light roux — this helps thicken the soup slightly.
Step 5: Add broth and simmer
Slowly pour in beef broth, stirring to combine. Bring to a simmer and cook uncovered for 25–30 minutes, allowing the flavors to meld. Taste and adjust seasoning with salt and pepper.
Step 6: Make the cheesy toasties
Preheat the oven broiler. Arrange bread slices on a baking sheet and brush lightly with olive oil or butter. Toast under the broiler for 1–2 minutes until golden. Remove, top each slice with shredded cheese, and return to the broiler until melted and bubbly.
Step 7: Assemble and serve
Ladle hot soup into bowls, top with one or two cheesy toasties, and sprinkle with fresh herbs. Serve immediately for maximum gooey deliciousness.
Tips for the Best Mushroom & Caramelized Onion Soup
- Caramelize slowly — patience builds rich flavor.
- Use real butter — adds sweetness and depth.
- Deglaze the pan — a splash of wine or broth lifts all those golden bits.
- Choose flavorful mushrooms — cremini or baby bella add more umami.
- Use good-quality broth — it’s the backbone of your soup.
- Add thyme — it pairs beautifully with both onions and mushrooms.
- Don’t skip the flour — it adds body without making the soup heavy.
- Broil cheese right before serving — so it stays melty and crisp.
- Use day-old bread — it holds up better when soaked in broth.
- Finish with a drizzle of olive oil — enhances the richness.
Variations & Substitutions
- Vegetarian Version: Use vegetable broth and omit flour (or use gluten-free).
- Creamy Option: Stir in ½ cup heavy cream for a luxurious finish.
- Add Wine: Deglaze onions with white wine or dry sherry for depth.
- Herb Twist: Add rosemary or sage for a heartier aroma.
- Cheese Choices: Try provolone, cheddar, or fontina instead of Gruyère.
- Add Meat: Top with crispy bacon or shredded rotisserie chicken.
- Low-Carb: Skip the toast and serve with a sprinkle of cheese directly on top.
- Extra Umami: Add a teaspoon of soy sauce or Worcestershire sauce.
- Vegan Option: Use vegan butter and dairy-free cheese.
- Add a Splash of Balsamic: Balances the sweetness of caramelized onions.
What to Serve With Mushroom & Caramelized Onion Soup
- Garlic bread or rustic baguette slices
- Simple green salad with lemon vinaigrette
- Grilled cheese sandwich
- Roasted vegetables or mashed potatoes
- Glass of red wine — Merlot or Pinot Noir pairs beautifully
Storage & Reheating Tips
To Store:
Refrigerate in an airtight container for up to 4 days.
To Freeze:
Cool completely, then freeze (without the toast) for up to 3 months.
To Reheat:
Warm on the stove over medium heat. Re-toast the bread and melt cheese fresh before serving.
Recipe FAQs
Can I use chicken broth instead of beef broth?
Yes, it will be slightly lighter but still flavorful.
Can I caramelize onions ahead of time?
Absolutely — store them in the fridge for up to 3 days.
Can I use a different type of mushroom?
Yes — try shiitake or portobello for extra depth.
Do I need to use flour?
It helps thicken the soup slightly, but you can skip it for a thinner broth.
Can I make it vegetarian?
Yes — use vegetable broth and dairy-free cheese.
Can I make this in a slow cooker?
Yes, caramelize the onions first, then simmer everything for 4–5 hours on low.
Why are my onions burning instead of caramelizing?
The heat is too high — reduce the temperature and stir often.
Can I add cream to the soup?
Yes, for a richer, more indulgent texture.
Can I make the toasties in advance?
Make the bread portion ahead, then melt cheese just before serving.
What cheese melts best?
Gruyère, Swiss, or mozzarella give that perfect stretch and golden crust.
Final Thoughts
This Mushroom & Caramelized Onion Soup with Cheesy Toasties is comfort food at its finest — a beautiful blend of earthy, savory, and sweet flavors wrapped in silky broth and finished with crispy, gooey cheese toasts.
It’s simple yet elegant, rustic yet refined — the kind of soup that warms you from the inside out. Serve it on a cold evening with a glass of wine, and you’ve got yourself the coziest dinner imaginable.