Step 1

Preheat the oven to 375 degrees F.

Step 2

Drizzle oil into a large pot of boiling salted water.

Step 3

Add the macaroni and cook according to the directions on the package, 6 to 8 minutes.

Step 4

Drain well.

Step 5

Meanwhile, heat the milk in a small saucepan, but don't boil it.

Step 6

Melt 6 tablespoons of butter in a large (4-quart) pot and add the flour.

Step 7

Cook over low heat for 2 minutes, stirring with a whisk.

Step 8

While whisking, add the hot milk and cook for a minute or two more, until thickened and smooth.

Step 9

Off the heat, add the Gruyere, Cheddar, 1 tablespoon salt, pepper, and nutmeg.

Step 10

Add the cooked macaroni and stir well.

Step 11

Pour into a 3-quart baking dish.

Step 12

Slice the tomatoes and arrange on top.

Step 13

Melt the remaining 2 tablespoons of butter, combine them with the fresh bread crumbs, and sprinkle on the top.

Step 14

Bake for 30 to 35 minutes, or until the sauce is bubbly and the macaroni is browned on the top.