Cook pasta per package directions and drain, reserving a small amount of pasta water.
In a small pot on medium heat, melt butter then remove from heat. Whisk 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
Double the recipe if desired. Bake in a 3 quart baking dish or 9"x13" casserole dish.