Preheat oven to 350 degrees and butter a 9X13 casserole dish.
Cook pasta in salted boiling water until al dente, about 2-3 minutes less than cooking time on box. Drain, reserving 1/2 C pasta water and set aside. Do not rinse the pasta.
In a large saucepan melt butter over low heat. Whisk in flour and cook about 2 minutes, whisking constantly, over low heat to cook the flour. Increase heat to medium and slowly add the milk, whisking and cooking until it thickens, forming a beschmel like sauce, about 3-4 minutes.
Once thickened, whisk in the dry mustard, salt , pepper and nutmeg. Turn heat to very low and add the Velveta cheese, stirring until melted. Turn off heat and add 2 C each of the cheddar and monetary jack cheeses, stirring until fully melted. If mixture seems too thick, whisk in some of the pasta water. Taste cheese sauce to make sure it has enough seasoning.
In the large pasta pot mix together the cooked pasta and cheese sauce. The mixture should seem quite wet at this point as the pasta will absorb the liquid during cooking. Pour mixture into the greased baking dish and smooth the top. Sprinkle remaining 1/2 C cheddar and Monterey jack cheeses over the top.
Bake about 30 minutes until bubbling and starting brown on the edges. Rest about 15 minutes before serving.
Enjoy!