COOK potatoes and garlic in boiling water in large saucepan 20 min. or until potatoes are very tender. Drain; return to saucepan. Add sour cream. Mash potatoes to desired consistency. Stir in 1/4 cup Cheddar.
HEAT oven to 375°F. Brown meat in large nonstick skillet. Stir in flour; cook and stir 1 min. Add vegetables, broth and ketchup; cook 5 min., stirring occasionally. Spoon into 8-inch square baking dish; cover with potatoes.
BAKE 18 min. or until heated through. Top with remaining Cheddar; bake 2 min. or until melted.