Cook pasta per package and drain, reserving a small amount of pasta water.
In a small pot, melt butter then remove from heat. Stir in flour until well combined.
Return to heat and add milk gradually. Whisk constantly until the sauce thickens. Remove from heat and add dry mustard and 1 cup of cheese, stirring until smooth. Season with salt and pepper.
Combine pasta and cheese sauce together. Add onion if using. If it is a little dry, mix in some reserved pasta water.
Add to casserole dish and sprinkle with remaining cheese. Bake 10-15 minutes, until bubbling.
Notes
*Substitute all purpose flour for whole wheat flour if needed.
Double the recipe if desired.
Bake in a 3 quart baking dish or 9"x13" casserole dish.