Vegetarian French Onion Soup Gratin

French onion soup gratin is defined by its deep, sweet onion base and the crisp, molten cheese-topped bread that turns a simple soup into a substantial dish. In this vegetarian version, yellow onions are slowly caramelised until jammy, then simmered with thyme, dry white wine and vegetable stock for a savoury, rounded broth without relying on meat stock. The toasted baguette and Gruyère topping gives the familiar gratin finish: crunchy at the edges, bubbling and rich in the centre. It is best served straight from the grill, while the cheese is golden and the bowls are still piping hot.
Cuisine style French bistro-style comfort food
Flavor profile Deeply savoury • Sweet caramelised onion • Nutty cheese
Techniques Slow caramelising • Simmering • Grilling
Cooking style Make-ahead friendly
Time commitment Slow-cooked
Key equipment Large pot • Oven-safe bowls • Grill

Why this recipe works

  • Slow-built flavour: Long, gentle caramelisation turns simple onions into a sweet, savoury base with real depth.
  • Balanced richness: Dry white wine lifts the sweetness of the onions and keeps the broth from tasting heavy.
  • Classic gratin finish: Toasted baguette supports the melted Gruyère, giving the soup its crisp, golden top.

Get the most out of this recipe

Timing notes

  • Most of the time is caramelising: The active work is modest, but the onions need slow, steady cooking to become jammy rather than sharp.
  • Serve straight after grilling: The gratin topping is best while the cheese is bubbling and the bread still has crisp edges.

Make-ahead tips

  • Make the soup base ahead: Cook and chill the onion soup without the bread or cheese, then reheat gently before adding the gratin topping.
  • Keep the topping separate: Store the soup base on its own so the baguette does not soften before grilling.
  • Freeze the base only: The soup base freezes well; add freshly toasted baguette and Gruyère after reheating.

Ingredient notes

  • Use yellow onions: Yellow onions caramelise well and give the soup a rounded sweetness without becoming too mild.
  • Choose a good melting cheese: Gruyère gives the topping a nutty flavour and smooth melt; Emmental makes a milder alternative.
  • Day-old baguette works well: Slightly stale bread holds its shape better under the cheese and absorbs less broth too quickly.

Common mistakes

  • Do not rush the onions: High heat can make the onions catch and turn bitter before they have properly caramelised.
  • Reduce the wine before adding stock: Letting the wine cook down concentrates its flavour and prevents the broth from tasting thin or raw.
  • Do not add the topping too early: Float the toast and cheese on the hot soup just before grilling to keep the gratin layer distinct.

Equipment you'll need

Ingredients
Instructions
Print
  1. Peel the onions, halve them through the root, then slice thinly. Set a large pot over medium heat and add the butter and olive oil.
  2. Add the onions and fine sea salt, then cook, stirring often, until deeply caramelised and jammy. Keep the heat at medium to medium-low so they brown slowly without catching.
  3. Lightly crush the black peppercorns and add them to the pot with the thyme. Pour in the wine and scrape up any browned bits from the base, then simmer until the wine is reduced by about half.
  4. Add the vegetable stock, bring to a gentle simmer, then cook for 15 minutes for the flavours to meld. Taste and adjust with a little more salt if needed, then remove the thyme sprigs.
  5. Heat the grill to high and toast the baguette until lightly golden on both sides. Grate the Gruyère cheese.
  6. Ladle the hot soup into oven-safe bowls set on a baking tray. Float the toasted baguette on top, pile over the Gruyère, then grill until bubbling and golden.
  7. Serve immediately while the gratin crust is crisp. Take care, as the bowls will be very hot.

Made this recipe? Rate it — a comment is optional.

Recipe notes

Tips

  • Caramelise the onions slowly: if they start to catch, lower the heat and stir more often to keep the flavour sweet rather than bitter.
  • For a cleaner finish, tie the thyme sprigs with kitchen string so they are easy to lift out before serving.

Substitutions

  • Replace Dry White Wine with the same amount of Vegetable stock for an alcohol-free version.
  • Replace Gruyère cheese with Emmental cheese for a milder, more elastic gratin topping.

Storage

  • Refrigerate up to 3 days
  • Store the soup (without baguette and Gruyère topping) in an airtight container. Reheat gently in a saucepan until piping hot, then top with freshly toasted baguette and Gruyère and grill to serve; freeze the soup base for up to 3 months.

Ratings & reviews

Liked this recipe? Share your feedback to support our cooking community.

Tried this recipe?

Choose 1–5 stars. A comment is optional.

If you leave a comment, use 10–500 characters.

Stay inspired

Discover new recipes, seasonal favourites and cooking inspiration every day. Follow JustRecipes on Pinterest and Spillt.

Vegetarian French Onion Soup Gratin
Image by JustRecipes
Pinterest

Recipe overview

  • Servings: 4 servings
  • Total time: 1h 30min (30min active)
  • Difficulty: Medium
  • Cuisine: French
  • Allergens:
    Gluten Milk Wheat
One pot Freezer friendly Kid friendly

Plan your meals and get a complete grocery list automatically.

Add to meal planner

Got ingredients left?

Find recipes using ingredients from this recipe or what you already have at home.

Find recipes with these ingredients →

Browse alphabetically

Looking for a specific recipe? Choose a letter below to jump directly to recipes starting with that letter or number.

Enjoy cooking with JustRecipes?

Add JustRecipes as one of your preferred Google sources to discover our recipes more easily across Google Search.

Add JustRecipes to Google

Every recipe on JustRecipes follows the same structured editorial process. Learn how recipes are created, validated, translated, illustrated, and continuously improved across the platform. How we create recipes.