Step 1: Boil the potatoes in a large pot of salted water until tender, about 15-20 minutes.
Step 2: While potatoes boil, brown the ground lamb in a large skillet over medium-high heat, draining excess fat if necessary.
Step 3: Add onions and carrots to the skillet and cook until softened, about 5-7 minutes. Stir in the garlic, thyme, and tomato paste.
Step 4: Pour in the beef broth and Worcestershire sauce. Simmer for 10 minutes until the sauce thickens, then stir in the peas.
Step 5: Drain the potatoes and mash with butter and milk until smooth. Season with salt and pepper.
Step 6: Spread the lamb mixture into a 9x13 inch baking dish. Top with the mashed potatoes, smoothing with a fork to create ridges.
Step 7: Bake at 400°F (200°C) for 25-30 minutes, or until the potato peaks are golden brown.