Place hamburger and onion in a medium saucepan on medium heat.
Break up and stir occasionally as hamburger browns (if hamburger has high fat content, drain excess fat before proceeding).
Meanwhile, heat water in microwave or on stove-top and preheat oven as box instructs.
Place butter in a medium casserole (1-½ qt) and add hot water (stir until butter is melted).
Add and mix potatoes, milk, hamburger, and sauce.
Bake uncovered as box instructs.