Start by preheating your oven to 350 degrees F. Cook your choice of pasta in boiling salted water until it's just al dente, usually a minute less than the package suggests.

Melt the butter in a large saucepan over medium heat. Add the flour and whisk to create a roux. Cook for 2 to 3 minutes until it smells toasty.

Gradually pour in the milk and half and half, whisking constantly to prevent lumps. Keep whisking until the mixture thickens, which should take about 5 minutes.

Turn the heat to low and start adding the cheeses (save 1/2 cup of cheddar for later). Add them slowly, one handful at a time, stirring until each batch is melted before adding more.

Season the cheese sauce with a pinch of salt, pepper, and the nutmeg. Give it a taste and add more seasoning if you think it needs it.

Combine the cooked pasta and crabmeat in a 9x13 inch baking dish or a 5-quart pot. Pour the cheese sauce over and stir to mix everything well. Sprinkle the top with the reserved cheddar.

Mix the crushed Ritz crackers with melted butter and Old Bay seasoning in a bowl. Once combined, spread this mixture over the mac and cheese.

Bake in the preheated oven for 30 to 40 minutes, or until the dish is heated through and the top is golden and bubbly.