Cook pasta according to package directions to the al dente stage. Drain and set aside.
In a large pot or dutch oven, melt the butter over medium heat.
When the butter is melted, add the flour, salt, and mustard, a little at a time, whisking continually.
When the flour and butter mixture is smooth, add the milk, a little at a time. Keep whisking to make sure you don't have any lumps.
Continue to cook for about two minutes or until the sauce is thickened and smooth. Test the mixture by coating the back of a spoon with the sauce. Run your finger across the sauce. If the line is thick enough that it stays put without dripping or running, it's done.
Add the cheese slowly to the sauce, about a cup at a time, and stir until melted. Once you have added all the cheese, add the cream cheese and stir until combined. Remove the pan from the heat.
Add the pasta to the sauce and mix well to combine.
Pour the macaroni mixture into a large 5-quart baking dish (11" x 15"), sprayed with no-stick cooking spray. Set aside.
Add Ritz crackers to your food processor and pulse until the crackers are crushed into fine crumbs. Add the melted butter and parmesan cheese and mix well to combine.
Top your casserole evenly with the crumb mixture and bake for 30-35 minutes or until the sauce is bubbly and the crumb topping is crispy and golden brown.
Notes
In a pinch, or if you prefer it, you can substitute just about any pasta you like or have on hand for the elbow macaroni. Good choices include penne, shells, farfalle, rotellini, cavatappi, fusilli, ziti, and rigatoni.If you don't have kosher salt and want to substitute table salt, use only half as much as the recipe calls for. Feel free to substitute sharp white cheddar, Monterey Jack, Gruyere, Manchego, Gouda, or any combination of hard cheeses you have on hand or like for the cheddar.If you don't want to use your food processor to crush the crackers, place them in a plastic bag and crush them with a rolling pin or even your hands until they are broken into fine crumbs.Leftovers will keep in a tightly covered container for three to five days in the refrigerator or up to two months in the freezer. To reheat in the oven, cover with aluminum foil and heat for 25-30 minutes. If frozen, allow it to thaw overnight in the fridge before reheating. You can make this casserole up to 24 hours ahead. Make it up to the point of putting the macaroni and cheese mixture in a casserole dish, and do not make the topping. Cover tightly and refrigerate. Before serving it, remove it from the refrigerator to allow it to come to room temperature. Make the topping as directed and then bake as directed. To make this recipe gluten-free, use gluten-free elbow macaroni, substitute one-fourth cup of cornstarch for the flour, and use gluten-free panko crumbs instead of the Ritz cracker crumbs.