Lightly grease shallow 1 1/2-quart casserole. In medium saucepan, bring water to a boil. Remove from heat. Add milk, salt and garlic; stir. Add potato flakes and sour cream; mix well.
Spoon potatoes into greased casserole. Sprinkle with onions and cheese. Broil 4 to 6 inches from heat for 30 to 60 seconds or until golden brown.