Published: Oct 27, 2020 · Modified: Jan 23, 2024 by Stephanie McKercher, RDN · This post may contain affiliate links · 2 Comments

Vegetarian French Onion Soup with Mushrooms

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This vegetarian French Onion Soup is made with chunky portobello mushrooms for meaty flavor and texture in a vegetable-based broth.

Vegetarian French Onion Soup with portobello mushrooms in a white soup crock on a wood board.

There's really no such thing as "soup season" in my world. Soup is the most comforting and satisfying meal. It's a good choice at any time of year.

I will say, though, warm crocks of French Onion Soup with melty cheese are especially delicious when you're in need of a warm-up. With year-round produce like onions and mushrooms, this is also something you can easily find the ingredients for no matter the season, even if it's the middle of winter.

Horizontal image of white soup crocks with mushroom onion soup.

Ingredients

Here's the grocery list with everything you'll need to make vegetarian French Onion Soup:

Oil to coat the pot while sautéeing (I use grapeseed oil)
Onions (I prefer mild, sweet onions, such as Vidalia, for this recipe)
Shallot (optional, but I like the sweet flavor it adds)
Portobello mushrooms (large caps)
Vegetable/mushroom broth (you can also make your own broth from vegetable scraps)
Fresh rosemary
Baguette or any type of sliced bread
Vegetarian Gruyere cheese (substitute with swiss, provolone, or mozzarella if you can't find Gruyere)
Fresh thyme leaves (optional for garnishing at the end)

For equipment, remember you'll also need oven-safe soup crocks if you want to melt the cheese on top of the soup under the broiler. (If you're not able to buy them in time, I'll explain how you can make French Onion Soup with regular bowls later on, too!)

Mushrooms, onions, rosemary, shallot, and vegetable broth on white marble.

Instructions

The first step for making vegetarian French Onion Soup is to sauté the onions. You can make the soup in a large stockpot or a Dutch oven. First, preheat the pot over medium heat, and then add the oil. Stir in the sliced onions and cook them for about ten minutes, stirring occasionally, or until they're soft and beginning to turn golden brown.

Once the onions are cooked, stir in the sliced shallot and chopped portobello mushrooms, and cook for about five minutes, or until the mushrooms turn dark brown.

Now it's time to add the liquids. Pour in vegetable broth (or mushroom broth) and water. Stir in a few sprigs of fresh rosemary for flavor. Then, cover the pot and bring the liquid to a boil. Once boiling, reduce the heat to low and simmer for ten minutes, or until the onions are fully cooked and the flavors all meld together.

Horizontal image of chopped portobello mushrooms and fresh rosemary sprigs on a wood cutting board.

Do you love French Onion Soup with melted cheese on top? Me too, it's the best. This is a quick and easy way to do it.

Preheat your oven broiler and spread sliced bread (I like to use a baguette) on a rimmed baking sheet. Transfer the baking sheet to the oven and toast under the broiler for a couple of minutes on each side. Remove the toasted bread from the oven and set aside.

Next, arrange oven-safe soup crocks on the baking sheet. Divide the cooked French Onion Soup between the crocks. Place a piece of toasted baguette and a slice of cheese on top of each bowl of soup.

Carefully return the baking sheet with the soup crocks to the oven and broil two minutes, or until the cheese is melted. Remove soups from the oven and garnish with fresh thyme leaves if desired.

Note that the soup crocks will be very hot when they come out of the oven. Serve the soup on plates or wait a few minutes before serving so they can cool down a little bit.

Closeup of French Onion Soup in a white crock with loaf of bread behind it.

Can you make this recipe ahead of time? How do you store it?

If you want to make this vegetarian French Onion Soup ahead of time, I recommend cooking the soup up until the point that you toast the bread.

Store the cooked French Onion Soup in an airtight container and refrigerate it for up to four days (You can also freeze the soup for up to three months.)

Once you're ready, divide the soup into crocks and toast the bread and melt the cheese immediately before serving.

Is Gruyere cheese vegetarian?

Some cheeses, including Gruyere, can be made with rennet, an enzyme from calves, which means it's technically not vegetarian. Some brands (such as Boar’s Head) make vegetarian Gruyere cheese without animal-derived rennet.

If you're not able to find suitable Gruyere cheese, you can substitute swiss, provolone, or mozzarella cheese instead. (Dairy-free cheese alternatives work great, too!)

Can you make this recipe without oven-safe soup crocks?

If you don't have oven-safe soup crocks, you can make this recipe with one simple change to the instructions. Cook the soup like normal and divide it between bowls. Top each with a piece of toasted bread and the sliced cheese. Working in batches, microwave each soup crock for 20 seconds, or until the cheese is melted.

Two white crocks of onion soup topped with toasted bread and melted cheese.

Ingredient Substitutions

Vegan/Dairy Free Variation: Omit cheese or substitute dairy-free cheese slices.
Gluten-Free Variation: Omit bread or substitute sliced gluten-free bread.
If you don't have portobello mushrooms, substitute baby bella or crimini mushrooms.
Instead of Gruyere cheese, use sliced swiss, mozzarella, or provolone.
If you don't have fresh rosemary sprigs, substitute one tablespoon of dry crushed rosemary.

Best Recipes to Make with Vegetarian French Onion Soup:

Two bowls of French onion soup in white crocks on a wood board.

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