Cook spaghetti, drain, and mix with spaghetti sauce. While spaghetti is cooking, melt butter, add onion, and cook until onion is soft. Stir in flour a nd slowly add evaporated milk and then water, whisking constantly, cook over me dium heat until thickened. Add 1/2 cup to 3/4 cup of Velveeta and Parmesan che ese. Stir until melted. Spread half of spaghetti mixture in the bottom of a 10x6x2" baking dish or 8"-9" square baking dish. Spread half of cheese mixture and half of remaining half of Velveeta over this layer. Spread remaining spaghetti mixture on top, and then remaining cheese sauce and remaining Velveeta. Bake at 350 for 15-20 mins.