To turn a ham and cheese into a bistro-ready Croque Monsieur, bring 1 cup of whole milk to a simmer in a saucepot. Mix together 1 tablespoon softened unsalted butter and 1 tablespoon flour to create a paste, and then whisk into the hot milk. Continue whisking until thickened, about 10 minutes, over medium heat. Season with salt and nutmeg. Pour a little béchamel over each sandwich, and top with more grated Gruyère. Broil until the top is bubbly and browned.
Heat ½ tablespoon butter in a saute pan until foamy. Add the ham in 2 piles and cook until warmed and lightly browned, 2 to 3 minutes per side. Meanwhile, brush the remaining butter on 1 side of the bread slices, and sprinkle lightly with salt. Divide the cheese between the un-buttered sides of the bread slices. Place 1/2 of the slices buttered-side down into the pan, adding more butter to the pan if needed. Lay the ham on top, and top with remaining bread slices. Place a heavy pan on top of the sandwiches to weight down, and cook until golden on both sides and cheese is melted, 2 to 3 minutes per side.