1. preheat oven to 400°F and grease a casserole dish

2. boil potatoes in salted water until tender then drain and mash with butter and milk

3. heat olive oil in a skillet and brown ground beef then drain excess fat

4. add onion and garlic to the beef and sauté until fragrant

5. stir in tomato paste worcestershire sauce broth thyme salt and pepper and simmer to thicken

6. mix in frozen vegetables and cook until heated through

7. spread beef mixture in the casserole dish and top evenly with mashed potatoes

8. score the potato surface with a fork and sprinkle cheese if using

9. bake for 20 to 25 minutes until the top is golden and the filling bubbles

10. let rest for 5 minutes before serving