Peel 4 small white onions per serving. Cut an "X" into the bottom of each onion and drop into a pot of boiling water. Boil for 5 minutes. Pour off water, add fresh water and boil again for 20 minutes. Strain. Place cooked onions into 7x11 inch casserole. Pour in 2 cups white sauce. Sprinkle with Parmesan cheese. Top with crushed Ritz crackers mixed with melted butter. Bake at 350 degrees for 30 minutes.