Preheat oven to 400°F (200°C)
Peel and chop potatoes
Boil potatoes in salted water until tender
Drain potatoes
Mash potatoes with butter, milk, salt, and pepper
Brown ground lamb or ground beef in a skillet
Add chopped onion, carrots, and garlic
Cook until vegetables soften
Stir in tomato paste and Worcestershire sauce
Add peas and corn
Season with salt, pepper, thyme, and rosemary
Simmer until filling thickens
Spread meat mixture into a baking dish
Spoon mashed potatoes over the top
Smooth the potato layer with a fork
Bake until the top is golden and filling is bubbling
Let cool slightly before serving