Bring a large pot of salted water to boil.
Add the macaroni and cook until tender.
This takes about 8-10 minutes.
Drain the pasta well in a colander.
Set it aside while you make the sauce.
Melt butter in a large saucepan over medium heat.
Add flour and whisk for 2 minutes.
This creates a roux for thickness.
Slowly add warm milk while whisking constantly.
Keep whisking until the sauce thickens.
This takes about 5 minutes.
Remove from heat and add both cheeses.
Stir until completely melted and smooth.
Add salt, pepper, and nutmeg. Mix well.
Pour the cooked macaroni into the cheese sauce.
Stir everything together until well combined.
Make sure every pasta piece gets coated.
Pour this mixture into your greased baking dish.
Spread it out evenly.
Arrange tomato slices on top of the mac and cheese.
They should cover most of the surface.
Mix breadcrumbs with melted butter in a bowl.
Sprinkle this mixture over the tomatoes evenly.
Preheat your oven to 375°F (190°C).
Bake the mac and cheese for 30-35 minutes.
The top should be golden and bubbly.
Let it rest for 10 minutes before serving.