Heat oven to 450°F. Make pie crust as directed on box for One-Crust Baked Shell using 9-inch deep-dish glass pie pan or 9-inch glass pie pan. Bake 9 to 11 minutes or until light golden brown. Cool while making mashed potatoes.
Meanwhile, in 3-quart saucepan, mix water, butter, salt, garlic powder and pepper. Heat to boiling. Remove from heat. Stir in potato flakes, 1/2 cup sour cream and the bacon bits.
Sprinkle 1/2 cup onions in bottom of cooled baked shell; sprinkle with cheese. Spoon and spread potato mixture evenly over cheese. Arrange tomato slices around edge, overlapping if necessary. Brush potato mixture and tomatoes with oil.
Reduce oven temperature to 400°F; bake 15 to 20 minutes or until thoroughly heated in center. Cool 5 minutes. To serve, sprinkle with 2 tablespoons onions. Cut into wedges; top each serving with 1 tablespoon sour cream.