In a large 12-inch skillet, over medium-high heat, add ground beef,onion, garlic, salt and pepper. Cook and crumble meat until browned. Drain grease.
Sprinkle flour evenly over meat mixture. Stir and cook 1 minute. Stir in tomato paste, broth, Worcestershire sauce and oregano. Bring to a boil, then reduce heat to medium-low. Stir in frozen peas, carrots and corn. Cook another 5 to 10 minutes or until heated through.
Pour meat mixture into the prepared baking dish. Spread mashed potatoes evenly over the top. Top potatoes evenly with cheddar cheese.
Bake, uncovered, for 35 minutes until casserole is hot and bubbly. Remove from oven and let stand 10 minutes before serving.