Heat oven to 375°F. Lightly butter a 3-quart baking dish.
Bring a large pot of water to a boil. Stir in 2 tbsp kosher salt.
Cook macaroni until just shy of al dente. Drain and set aside.
Melt 7 tbsp butter in the empty pot over medium heat.
Whisk in flour and cook, whisking, for 2 minutes to make a roux.
Slowly pour in warm milk, whisking, until smooth. Simmer until thick, 4–5 minutes.
Season with 1 tsp kosher salt, pepper, and nutmeg. Take off heat.
Stir in cheddar and Gruyère until fully melted and creamy.
Fold the drained macaroni into the cheese sauce until coated.
Melt remaining 3 tbsp butter. Toss with bread crumbs to moisten.
Spread pasta in the dish. Add tomato slices over the top, if using.
Cover evenly with buttered crumbs. Bake until bubbling and golden, 25–30 minutes.
Rest 5 minutes before serving for cleaner scoops.
Notes
Try swapping half the Gruyère with smoked gouda for a subtle smoky note. For make-ahead, assemble without baking, cover, and refrigerate up to 24 hours; bake 10–15 minutes longer from cold. Leftovers reheat well, covered, at 325°F with a splash of milk.This recipe is an original creation inspired by classic Easy Mac and Cheese Recipes from Ina Garten flavors. All ingredient ratios and instructions are independently developed.