Heat oven to 400°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray.
In 12-inch skillet, cook beef over medium-high heat 7 to 9 minutes, stirring occasionally, until beef is browned. Add onions, carrots and garlic; cook 4 to 6 minutes, stirring frequently, until softened. Add flour, salt and pepper; cook 1 minute, stirring frequently.
Stir in milk; heat to boiling over medium-high heat. Cook until thickened, stirring frequently. Remove from heat; stir in 1 cup of the cheese. When cheese has melted, stir in frozen peas; transfer mixture to baking dish.
Make mashed potatoes as directed on box. Drop potatoes by large spoonfuls over mixture in baking dish. Top with remaining ½ cup cheese. Bake 20 to 25 minutes or until bubbly and lightly browned. Let stand 10 minutes before serving.
