Start by bringing a large pot of water to a boil. Salt it generously until it resembles the sea. Add the macaroni and cook until al dente. This usually takes about 6-8 minutes. Drain and set aside but do not rinse. Rinsing washes away some of that starchy goodness, which helps the cheese sauce cling better.

In a large saucepan over medium heat, melt 6 tablespoons of butter. Once it's melted, add the flour, stirring continuously to create a roux. You want this to cook for about 2 minutes—just long enough for the raw flour taste to disappear. Slowly add in the warmed milk, continuing to stir. Cook for about 5 minutes or until the mixture thickens. It should coat the back of a spoon.

Remove the sauce from the heat. Stir in the Gruyère and Cheddar until melted. This is where the magic happens. The aromas filling your kitchen will have you chomping at the bit.

Now, add the cooked pasta into the cheese sauce. Stir until the pasta is well coated. Feel free to add that optional Dijon mustard for some zing and nutmeg for warmth. Season the mixture with salt and pepper to taste.

Pour the macaroni and cheese into a greased 3-quart baking dish. Arrange the sliced tomatoes over the top, then sprinkle the remaining cheese and breadcrumbs. For a golden crust, drizzle the last 2 tablespoons of melted butter over the top.

Preheat your oven to 375°F (190°C). Place the dish in the oven and bake for about 30-35 minutes or until the top is golden brown and bubbling.