Potatoes au gratin

Heat the oven to 350F.

Peel the potatoes, rinse if dusty, and slice them about an eighth of an inch thick so they cook evenly.

Halve the onion and cut it into thin slices.

Toss the potatoes in a bowl with the salt and black pepper, or season between the layers instead.

Shingle the potato slices into the baking dish, laying onion slices in between every few potatoes, and fill any gaps with onion at the end.

Make ahead: bake an hour, cool completely, refrigerate, and bake 40 minutes a day or two later with an extra quarter cup of cream or milk over the top.

Shred the Gruyere and the cheddar.

Melt the butter in a medium pot over medium heat.

Sprinkle in the flour while whisking and cook, whisking constantly, for 2 minutes until the flour is no longer raw, takes on colour and the butter begins to brown.

Drizzle in the warm milk while whisking constantly, then cook until it bubbles and thickens slightly.

Off the heat, stir in the cheese until melted; adding it over heat can break the sauce.

Stir in the salt and black pepper.

Pour the sauce over the potatoes, smooth it out for full coverage and cover with foil.

Bake covered at 350F for one hour.

Remove the foil and bake 20 to 30 minutes more, until the top is golden, then cool slightly.