Mary Berry Leek and Potato Gratin

I made this one during a Sunday roast prep sprint—I had exactly 45 minutes before guests arrived and a fridge full of random odds: leeks, half a block of cheddar, and some leftover potatoes. I figured, “How hard can a gratin be?”

Well, I overboiled the potatoes (they practically mashed themselves), my sauce went lumpy because I added cold milk in a rush, and the top didn’t brown the way I’d hoped.

So I tried again. Slowed down. Heated the milk first, cooked the veg just enough, grated the cheese fresh—and suddenly it became a regular in my side dish rotation. It’s creamy, cheesy, and the leeks give it just enough bite to make it feel grown-up.

Here’s the version that actually works.

WHY THIS ONE WORKS SO WELL

This is comfort food with a little polish. The combination of Gruyère and mature Cheddar gives it that perfect mix of melt and tang, and the Dijon mustard in the sauce cuts through the richness just enough.

Blanching the veg before baking means everything finishes at the same time, and heating the milk makes for a lump-free sauce (I learned that the hard way).

INGREDIENTS + WHY THEY MATTER

• Potatoes (500g) – Go waxy or all-purpose (like Maris Piper). Hold their shape after parboiling.
• Leeks (2 large) – Sweet, tender, and give a bit of green and oniony flavour.
• Butter + Flour + Hot Milk – Classic white sauce base. Hot milk = no lumps.
• Dijon Mustard (2 tsp) – Lifts the whole dish. Don’t skip it.
• Gruyère + Cheddar (75g each) – Gruyère melts beautifully, Cheddar brings sharpness. A dream combo.

MAKING IT YOURS (WITHOUT RUINING IT)

• No Gruyère? Use more Cheddar or swap for Emmental.
• Want it meatier? Add chopped ham or pancetta when you mix in the veg.
• No mustard? A splash of lemon juice or a pinch of cayenne gives a similar lift.

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

WHAT WENT WRONG	WHY IT HAPPENS	HOW TO FIX IT
Lumpy sauce	Added cold milk too fast	Always warm the milk before adding
Mushy potatoes	Boiled too long before baking	5 mins max, then drain well
Greasy top	Used pre-shredded cheese	Grate it fresh—it melts smoother

HOW TO MAKE MARY BERRY’S LEEK AND POTATO GRATIN

1. Preheat oven to 200°C (180°C fan). Grease a 1.75-litre dish with butter.
2. Parboil potatoes: Boil cubes for 5 mins in salted water. Add leeks, boil 5 more mins. Drain and rinse under cold water. Drain again.
3. Make sauce: Melt butter, stir in flour for 30 sec. Gradually whisk in hot milk until thick and smooth.
4. Add flavour: Stir in mustard, salt, pepper, and one-third of each cheese.
5. Combine: Add potatoes and leeks to the sauce and mix to coat.
6. Assemble + bake: Pour into dish, top with remaining cheese. Bake 30–35 mins until bubbling and golden.

TIPS FROM MY KITCHEN

• I use a balloon whisk for the sauce—helps prevent any sneaky lumps.
• If making ahead, I cover and chill it before baking, then add 10 mins to oven time.
• Don’t skip the cold rinse—it stops the veg overcooking in the oven.
• A few thyme leaves stirred into the sauce gives a lovely herby edge.

STORAGE + SERVING

• Fridge: Keeps 3 days in a sealed container.
• Reheat: 180°C oven for 15 mins or microwave in 3-min bursts.
• Freezer: Not ideal—sauce can split.
• Serve With: Roast chicken, sausages, or just a green salad and a spoon.

FREQUENTLY ASKED QUESTIONS

Q: Can I make this ahead of time?
A: Yes—assemble it, then chill unbaked for up to 8 hours. Add 10 mins to the bake time.

Q: What’s the best cheese substitute if I don’t have Gruyère?
A: Emmental or extra mature Cheddar works well.

Q: Can I use cream instead of milk?
A: You can—but it’s already rich. I’d go half cream, half milk at most.

Q: Why is my sauce grainy?
A: Likely the milk was too cold or added too fast. Heat it gently and whisk steadily.

Try More Recipes:

• Mary Berry Leek and Potato Soup Recipe
• Mary Berry Leek and Potato Gratin
• Mary Berry Fish Pie With Crushed Potatoes​