Mary Berry Onion Sauce Recipe

The first time I made this, it was a last-minute save for a dry-ish roast lamb. The mint jelly had vanished (likely eaten straight from the jar—no one’s admitting it), and I needed something warm, creamy, and a little nostalgic. Onion sauce it was.

Now, I’ll be honest: I thought it would be bland. White sauce and boiled onions? But once I let the onions go really soft, added a splash of cream, and seasoned it like I meant it—something magical happened. It became a proper “gravy alternative” that didn’t just sit on the plate—it lifted everything.

WHAT MAKES THIS RECIPE SPECIAL

This isn’t just a beige blob on the side of your Sunday roast. It’s the underdog sauce with quiet confidence. Here’s why:

• Gently cooked onions give a sweetness that’s mellow, not sharp.
• Butter + flour base makes it luxuriously smooth.
• Hot milk + slow stir = no lumps, ever
• And the best part? It’s endlessly customisable—rich with cream, sharp with mustard, or even blended silky if you’re in that mood.

INGREDIENTS + WHY THEY MATTER

• Butter (25g) – Forms the flavour base and makes everything rich.
• Medium Onion (1) – Finely chopped is key. You want softness, not chunks.
• Flour (25g) – Thickens the sauce. Skip this and it’ll stay watery.
• Hot Milk (300ml) – Hot milk blends smoothly and speeds things up.
• Salt + Pepper – Season generously. Onion needs it to sing.

MAKING IT YOURS (WITHOUT RUINING IT)

• Creamier? Swap half the milk for double cream. Luxurious.
• Low-FODMAP? Use the green part of spring onions or leek tops instead.
• No flour? I’ve made it with 1 tsp cornflour mixed with cold milk—it thickens fine, but needs more whisking.
• Want bite? Add a pinch of English mustard powder for a sharp twist.

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

What Went Wrong	Why It Happens	How to Fix It
Lumpy sauce	Added cold milk all at once	Use hot milk and add gradually while stirring
Raw flour taste	Didn’t cook the flour long enough	Stir for a full minute before adding milk
Too thin	Not enough flour or too much milk	Let it boil gently to thicken, or add more flour (pre-mixed)

HOW TO MAKE MARY BERRY’S ONION SAUCE

1. Cook the Onion
Melt butter in a small saucepan over high heat. Add finely chopped onion and cook for 1 minute, stirring. Reduce heat, cover, and cook gently for 20 minutes until soft—but not browned.

2. Make the Base
Sprinkle in flour and stir for 1 minute until fully combined and just starting to bubble.

3. Add the Milk
Gradually pour in the hot milk, stirring constantly. Bring to a gentle boil and let it bubble until smooth and thickened.

4. Season + Serve
Add salt and freshly ground pepper to taste. Serve hot with lamb, leeks, or mash. (A swirl of cream never hurts.)

TIPS FROM MY KITCHEN

• I chop the onion almost to a mince—it melts into the sauce better that way.
• I warm the milk in the microwave for 30 seconds before adding. Makes everything smoother.
• If I’m serving it over leeks, I blend the sauce—it clings better and looks elegant.

STORAGE + SERVING

• Fridge: Store for up to 2 days in a sealed container.
• Reheat: Gently over low heat with a splash of milk to loosen.
• Serve with: Roast lamb, boiled ham, steamed leeks, or mashed potatoes.

FREQUENTLY ASKED QUESTIONS

Q: Can I freeze onion sauce?
A: Technically yes, but the texture can split. It’s better made fresh or kept chilled for a couple of days.

Q: Can I make it ahead?
A: Absolutely. Reheats well on the hob—just stir gently and add a bit of milk.

Q: Can I use red onion?
A: I wouldn’t. It sweetens differently and turns the sauce pinkish—not what you want here.

Q: What’s the best way to blend it?
A: A stick blender works perfectly. Do it off the heat once thickened.

Try More Recipes:

• Mary Berry Stuffed Chicken Thighs With Lemon Sauce
• Mary Berry Cannelloni With Double Cheese Sauce
• Mary Berry Sea Bass With Creamy Lemon Sauce