In 12-inch skillet over medium heat, in hot oil, cook onion and garlic 5 minutes or until softened, stirring occasionally. Remove to plate.
In same skillet over medium-high, cook ground beef until well browned on all sides, stirring frequently. Add flour; cook 1 minute. Add stewed tomatoes and thyme. Over high heat, heat to boiling; reduce heat to low. Simmer uncovered 10 minutes to blend flavors, stirring occasionally. Stir in peas and carrots.
Preheat oven to 450°F. Spoon mixture into greased deep-dish pie plate or baking dish. Toss sliced potatoes with butter. Arrange potatoes in center of casserole, overlapping slightly. Bake 10 to 15 minutes or until potatoes are lightly golden.