Preheat oven to 375°F (190°C). Cook macaroni in salted water until just shy of al dente. Drain, reserving 1/4 cup pasta water.

In a saucepan, melt butter over medium heat. Add flour, whisking continuously for 1-2 minutes until the mixture is smooth and lightly golden.

Gradually add milk to the roux, whisking constantly until the sauce thickens and coats the back of a spoon.

Remove from heat and stir in grated cheddar and gruyere until melted and smooth. Season with salt and pepper.

Combine cooked pasta with cheese sauce, adding reserved pasta water to reach desired consistency.

Transfer mixture to a greased 9x9-inch baking dish. Mix breadcrumbs with olive oil and sprinkle evenly over the top.

Bake for 20 minutes, or until the top is golden brown and crisp. Listen for a sizzle as the sauce bubbles.

Let the dish rest for 5 minutes before serving. The sauce will thicken slightly, enhancing its creamy texture.