Mary Berry French Onion Soup With Mustard Cheese Croûtes

The first time I made this, I had no intention of making it “fancy.” I just had too many onions and a craving for something warm and brothy. But of course, Mary took it to another level. Add a splash of wine, some slow-cooked caramelisation, and top it with Dijon-glazed cheesy croûtes—and suddenly, it’s not just soup. It’s dinner worth sitting down for.

That said, I nearly ruined the first batch by rushing the onions. Rookie move. If there’s one tip to take from me: slow down. Let them go golden, soft, and sweet—don’t turn the heat up high and walk away.

And those croûtes? Toast the bread first or you’ll end up with soggy bottoms (and not the good kind).

WHY THIS ONE WORKS SO WELL

• Slow-cooked onions become meltingly sweet and savoury—worth the wait.
• A splash of white wine lifts the broth and adds depth.
• Gruyère + mustard on a toasted baguette = next-level soup topping.
• Easily adaptable for vegetarians with a simple stock swap.

INGREDIENTS + WHY THEY MATTER

FOR THE SOUP

• Butter + olive oil – Butter gives richness; olive oil prevents burning.
• White onions (6 large) – Sliced thin for quicker, more even caramelisation.
• Caster sugar (1½ tbsp) – Helps coax out sweetness and get that golden colour.
• White wine (200ml) – Adds brightness and balances the richness.
• Beef or veg stock (2L) – A proper stock makes all the difference.
• Bay leaves + rosemary – Adds herbal warmth to the background.
• Salt + pepper – Season in stages, especially with homemade stock.

FOR THE CROÛTES

• Thin baguette (½) – Sliced thick or thin depending on how crispy you want them.
• Dijon mustard – A punchy contrast to the sweet soup.
• Gruyère cheese (25g) – Melts perfectly and adds a nutty finish.

MAKING IT YOURS (WITHOUT RUINING IT)

• Swap the wine: Use dry cider or even a splash of sherry if that’s what you have.
• Make it veggie: Use a rich vegetable stock and double the herbs.
• Try different cheese: Comté, Emmental, or even sharp Cheddar if you’re out of Gruyère.
• Add depth: A splash of Worcestershire sauce or soy sauce adds umami if your stock is bland.

MISTAKES I’VE MADE (AND HOW TO AVOID THEM)

WHAT WENT WRONG	WHY IT HAPPENS	HOW TO FIX IT
Onions didn’t caramelise	Heat was too high or time too short	Cook low and slow—at least 30 minutes total
Croûtes went soggy	Bread wasn’t toasted first	Always toast one side before adding toppings
Soup tasted flat	Stock wasn’t flavourful enough	Use wine + herbs and season as you go

HOW TO MAKE MARY BERRY’S FRENCH ONION SOUP

CARAMELISE THE ONIONS
Melt butter and oil in a deep pan. Add sliced onions and fry for 2–3 minutes on high. Lower heat, cover, and cook for 10 minutes.

BUILD FLAVOUR
Remove the lid, stir in caster sugar, salt, and pepper. Fry for another 2–3 minutes on high, then reduce heat and cook uncovered for 15 minutes, stirring now and then, until deep golden.

ADD WINE AND STOCK
Pour in the white wine, let it bubble for 2–3 minutes to cook off the alcohol. Add the stock, bay leaves, and rosemary. Simmer for 10 minutes.

MAKE THE CROÛTES
Slice the baguette. Toast one side under a grill. Flip, spread with mustard, top with grated cheese, and grill until golden and bubbling.

SERVE
Remove the bay and rosemary from the soup. Taste and adjust seasoning. Ladle into warm bowls and float a cheesy croûte on top of each.

TIPS FROM MY KITCHEN

• I sometimes double the croûtes—one in the soup, one on the side for dipping.
• Let the onions go past “soft” to properly golden—it’s where the magic is.
• If you want it richer, stir in a splash of cream before serving (not traditional, but lush).

STORAGE + SERVING

Fridge: Soup keeps for up to 4 days in an airtight container.
Freezer: Freeze soup (no croûtes!) for up to 3 months.
Reheat: Gently on the stove until hot. Grill fresh croûtes separately.
Serve with: Extra mustard cheese toasties or a green salad with vinaigrette.

FREQUENTLY ASKED QUESTIONS

Q: Can I make the croûtes ahead of time?
A: Yes—toast and top them, then reheat under the grill just before serving.

Q: Can I skip the wine?
A: Technically yes, but it adds depth. Replace with a bit of cider vinegar or lemon juice for brightness.

Q: What’s the best onion to use?
A: White onions are ideal, but yellow works too. Just avoid red—they’re too sharp for this.

Try More Recipes:

• Mary Berry Honey-Glazed Parsnip Soup
• Mary Berry Japanese Noodle Soup Recipe
• Mary Berry Game Soup