Melt butter in large pot. Add onion and cook until transparent. Add salt, pepper, sugar, and parsley. Slowly add flour, to butter mixture. Use whisk to keep mixture smooth. Slowly add milk, to the rue mixture. Mixture will begin to have a thick consistency. Continue whipping as you slowly add cubes of Velveeta cheese. Add more milk, in small amount if necessary. Consistency will be thick, put pourable. Continue adding cheese until all is melted in the sauce. Be careful not to scorch mixture, will burn easily.
Meanwhile, cook macaroni as directed on package. Drain and put in baking dish. Add cheese sauce to baking dish coating all noodles. Sprinkle with shredded cheese.
Bake covered for 35 minutes at 350 degrees F. Uncover and bank an additional 10 minutes or until the top is golden brown.