Heat oven to 450°F. Heat mashed potatoes according to package directions.
Meanwhile remove beef pot roast from container; reserve gravy or au jus for another use. Shred pot roast in medium bowl with 2 forks; stir in thyme.
Spray 8 x 8-inch glass baking dish with cooking spray. Spread 1/2 of potatoes over bottom of dish; sprinkle with 1/2 cup cheese. Top evenly with shredded beef and vegetables; sprinkle with 1/2 cup cheese. Spread remaining potatoes over the top; sprinkle with remaining 1/2 cup cheese.
Bake, covered, in 450°F oven 15 minutes. Uncover; continue baking 10 to 12 minutes or until heated through and edges begin to brown. Let stand, loosely covered, 10 minutes before serving.