The potatoes here stay loose. There is no egg in the sauce, so the slices bake soft in a pale, spoonable Gruyère cream rather than setting into something you cut into wedges.
A simple butter and flour sauce with milk, cream, garlic powder and thyme carries most of the cheese. The rest goes on top for the last stretch in the oven, when the dish is uncovered to brown.
Ingredient Notes
- Yukon Gold potatoes
- Two pounds, scrubbed and sliced 1/8 inch thick; a mandoline keeps them even.
- Gruyère and Parmesan
- A cup and a half of Gruyère and half a cup of Parmesan, split between the sauce and the top.
- Fresh thyme
- One teaspoon of leaves in the sauce, with more scattered over at the end.
How to Make It
Oven at 375°F, potatoes shingled
Heat the oven to 375°F, butter a 2-quart oval baking dish and arrange the potato slices in overlapping rows.
Butter, flour, milk and cream
Melt the butter in a saucepan over medium heat, whisk in the flour for 1 minute, then gradually whisk in the milk and cream.
Seasoned, thickened and cheesed
Season with salt, pepper and garlic powder and simmer 3 to 4 minutes. Off the heat, stir in 1 cup Gruyère, 1/4 cup Parmesan and the thyme.
Sauce poured over, baked covered
Pour the pale sauce evenly over the potatoes, cover and bake for 35 minutes.
Uncovered with the rest of the cheese
Uncover, top with the remaining Gruyère and Parmesan and bake 20 to 25 minutes more until tender and blistered. Rest 10 minutes and add thyme.
Two Stages in the Oven
The bake is split so the potatoes soften before the cheese browns.
Covered bake35 minutes
The lid traps steam and cooks the sliced potatoes through in the sauce.
Uncovered with cheese20 to 25 minutes
The reserved Gruyère and Parmesan melt and blister on the exposed top.
Rest10 minutes
The sauce thickens as it cools slightly, so portions hold together on the plate.
Why This Recipe Works
- Slicing the potatoes 1/8 inch thick lets them cook through in the sauce in about an hour.
- A lightly thickened roux sauce clings to every slice instead of pooling at the bottom.
- Baking covered first steams the potatoes tender before the uncovered stretch browns the top.
- Gruyère and Parmesan together give a nutty sauce and a top that blisters rather than turning rubbery.
Serving Ideas
Spoon it straight from the oval dish so each portion gets soft slices from underneath and browned cheese from the top.
Success Tips
- Keep the gratin sauce at a gentle simmer so the milk and cream thicken without catching on the pan.
- Tuck any stray potato edges under the sauce before covering so they cook instead of drying out.
- Test the centre slices of the gratin with a knife; they should offer no resistance before you uncover.
Storage and Reheating
Refrigerate
Refrigerate leftovers promptly in a covered dish and reheat until the sauce is bubbling again.
Frequently Asked Questions
Will it set firm enough to slice?
No. There is no egg, so it stays creamy and is served with a spoon.
How do I know the potatoes are done?
A knife slides through the thickest slices easily and the sauce bubbles around the edges.
