Boil potatoes in salted water until tender; drain and mash with butter and milk.
Heat a little oil in a large skillet or Dutch oven over high heat. Add the lamb and brown well, breaking it up as it cooks. Spoon off excess fat, leaving a few tablespoons behind.
Add onion and carrot to the rendered fat and cook until softened; add garlic and cook 1 minute.
Stir in tomato paste and cook 1 minute. Add red wine and simmer until nearly evaporated, scraping up the browned bits.
Add stock, thyme, bay leaf, and Worcestershire; simmer 20 minutes until thick. Discard the bay leaf and stir in the peas.
Spread meat mixture in a baking dish and top evenly with mashed potatoes.
Bake at 400°F (200°C) for 25–30 minutes until the top is golden.
Rest 15–20 minutes before serving so the filling sets.
Pairings
Wine:
Rioja Reserva – savory oak and leather, medium tannin, a classic with lamb; Côtes-du-Rhône – grenache/syrah warmth and black pepper against the thyme
Beer:
ESB – the pub classic; malt sweetness meets the carrots and peas; Stout – roasted depth, best if you swap the wine for a splash of it