Preheat the oven to 350 degrees F. Cook the pasta in salted water according to the package directions, until it’s al dente.

In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook to create a roux. Cook for 2 to 3 minutes, stirring often, until the mixture is fragrant.

Slowly stream in the milk and half and half, whisking the entire time. The slower you stream it in, the easier it will thicken and come together. Continue whisking until the milk is smooth and thickened, about 5 full minutes.

Reduce the heat to low. Add all the grated cheeses except for ½ cup sharp cheddar, 1 handful at a time, stirring completely until melted before adding the next handful. Do this until all the cheese has been added.

Stir in a pinch of salt and pepper along with the freshly grated nutmeg. Taste and season the cheese sauce more if needed.

Place the pasta in a baking dish - either a 9x13 inch baking dish or a 5 quart pot. Stir in the lump crabmeat. Pour the cheese sauce on top and stir to mix it all in. Top with the remaining grated cheddar.

In a bowl, stir together the crushed ritz crackers with the melted butter and old bay. Stir until combined. Sprinkle all over the top of the mac and cheese.

Bake for 30 to 40 minutes, until warm and bubbly and cheesy! Serve immediately.