Preheat oven to 400°F (200°C)
Cook 1 to 1½ lb ground lamb or beef in a large skillet over medium-high heat until browned
Add 1 diced onion and cook 3 to 5 minutes
Add 2 to 3 minced garlic cloves and cook 30 to 60 seconds
Stir in 2 tbsp tomato paste (optional)
Add 1 to 2 tbsp Worcestershire sauce (optional)
Add 2 tbsp flour (optional for thickening)
Pour in 1 cup beef broth and stir until slightly thickened
Stir in 1 to 2 tsp dried thyme or 2 to 3 tsp fresh thyme
Stir in salt and black pepper to taste
Add 2 to 3 cups mixed vegetables (peas, carrots, corn), thawed or frozen
Transfer filling to a baking dish (about 9×13 in or similar)
Prepare mashed potatoes:
Boil 2 to 2½ lb potatoes (peeled or unpeeled) in salted water until tender, 15 to 20 minutes
Drain and mash with 4 to 6 tbsp butter and ½ to 1 cup milk (or cream)
Season with salt and pepper
Spread mashed potatoes evenly over the filling
Fork the top to create ridges (optional)
Bake 20 to 30 minutes until hot and bubbling
Optional: Broil 1 to 3 minutes to brown the top
Rest 5 to 10 minutes before serving