Preheat the oven to 375 degrees.
Cook the macaroni in salted boiling water per package instructions until just al dente. Drain.
Make the cheese sauce while the pasta is cooking by heating 2 cups of milk in the microwave for 1 minute (or in a small saucepan over medium heat), until hot but not boiling.
Heat 3 tablespoons of butter in a large saucepan over medium heat until melted.
Add the flour and cook, whisking constantly, for 2 minutes.
Slowly add the warm milk to the flour mixture while whisking.
Cook, stirring often, for 2 minutes, until thickened and smooth.
Off the heat, add the shredded Gruyere and sharp white cheddar cheese, and nutmeg.
Season with sea salt and freshly cracked pepper, to taste. Mix until well combined and creamy.
Add the macaroni and stir well.
Pour into an 8-inch baking dish.
Make the bread crumbs by placing the French bread in a food processor and pulse until bread crumbs.
Heat the remaining 1 tablespoon of butter in a small skillet over medium-high heat.
Add the bread crumbs then season with sea salt and freshly cracked pepper, to taste.
Cook, stirring often, for 2 minutes, until evenly coated with the butter.
Finish by placing the sliced tomatoes on top of the macaroni and cheese then sprinkle the bread crumbs evenly on top.
Place into the oven and bake for 30-35 minutes, or until the sauce is bubbly and the crumbs are golden brown on top.
Serve immediately. Enjoy.