Holiday ham and potato casserole

Saute onions in butter over medium heat with a big pinch of salt for a few minutes until just translucent.

Stir in flour and cook, stirring, about two minutes to take off the raw starchy edge (a roux).

Add cold heavy cream and whisk; cold liquid into hot roux gives no lumps. Whisk in cold milk.

Raise to medium-high and cook, stirring occasionally, adding salt, black pepper, cayenne and grated nutmeg, until it simmers and thickens significantly.

Off the heat, whisk in a dash of Worcestershire sauce and fresh thyme leaves; reserve.

Slice the potatoes on a mandolin, using the guard, about an eighth of an inch thick, one layer's worth at a time, keeping them in cold water.

Spread about 25% of the sauce in a large deep casserole dish; shingle a layer of well-drained potatoes and salt generously; drizzle about a third of the remaining sauce; add grated cheese and about half the ham.

Add a second salted layer of potatoes, a little less than half the remaining sauce, cheese and the rest of the ham, pressing each layer; finish with the last potatoes, all the remaining sauce and the last of the cheese.

Bake in the center of a 350F oven about an hour and a half, until browned and bubbling and a knife slides into the potatoes with no effort.

Rest 15 to 20 minutes before cutting.