1. Preheat oven to 375°F (190°C) and grease a medium baking dish.

2. Heat a skillet over medium-high heat, add ground beef and cook until browned, then drain excess fat.

3. Add diced onion, garlic, and mixed vegetables to the skillet and cook until vegetables soften.

4. Stir in tomato paste or gravy mix, paprika, garlic powder, onion powder, thyme, salt, and pepper until well combined.

5. Spread the beef mixture evenly in the prepared baking dish.

6. Prepare mashed potatoes by mashing potatoes with butter and milk until creamy, then stir in cheddar cheese if using.

7. Spread mashed potatoes over the beef layer and smooth the top with a spatula.

8. Bake uncovered for 20 to 25 minutes until the top is lightly golden.

9. Let the casserole rest for five minutes before serving.

10. Store leftovers in an airtight container in the refrigerator for up to four days or freeze for up to three months.