FOR THE BÉCHAMEL
4 tablespoons butter
¼ cup all-purpose flour
2 cups whole milk
1 ½ teaspoons salt
½ teaspoon ground white pepper
Pinch of freshly grated nutmeg
FOR THE CASSEROLE
Olive oil, as needed
1 large (18 to 20 ounces) eggplant, peeled and sliced into 1/4-inch-thick rounds
freshly ground black pepper
9 ounces merguez sausage
12 ounces fontina cheese, grated
6 ounces Parmesan cheese, grated
For the béchamel: In a small saucepan over low heat, melt butter. Add flour and whisk until pale golden, about 5 minutes.
Add milk, salt, white pepper and nutmeg, and whisk to combine thoroughly. Cook, whisking frequently, until thickened and smooth, 15 to 20 minutes. Remove from heat and set aside.
For the casserole: Preheat oven to 400 degrees. Spread eggplant slices in a single layer on a baking sheet, and thoroughly coat both sides with olive oil and a sprinkling of salt and black pepper. Roast until fully cooked, 12 to 15 minutes, then remove from heat but do not turn off oven. Meanwhile, in a medium skillet over medium-low heat, sauté merguez until browned and fully cooked. Remove from heat and slice into 1/4-inch-thick rounds.
Oil an 8-by-11, 2-inch-deep baking dish. Spread one-third of béchamel in baking dish. Top with half the eggplant, then half the fontina and half the merguez. Coat with half of the remaining béchamel. Top with remaining eggplant, fontina and merguez. Spread with remaining béchamel and the Parmesan.
Bake until hot, bubbling and lightly browned, 12 to 15 minutes. Allow to rest for a minute or two, then serve.