Place cubed potatoes in a large pot of cold salted water. Bring to a boil and cook for 15-18 minutes until fork-tender. Drain thoroughly and return to the hot pot.

Mash potatoes with 4 tablespoons butter, warm milk, 1/2 cup shredded cheddar cheese, salt, and pepper until velvety smooth. Cover to keep warm.

Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. Add ground lamb and beef, breaking up with a wooden spoon until browned (about 6-7 minutes). Drain excess rendered grease if desired.

Add diced onion and carrots to the skillet. Cook for 5 minutes until softened. Stir in minced garlic, tomato paste, minced rosemary, and thyme, cooking for 1 minute until fragrant.

Sprinkle flour over the meat and vegetables, stirring for 1 minute to coat. Slowly pour in beef bone broth and halal Worcestershire sauce, scraping up any caramelized bits from the skillet bottom. Simmer for 5-7 minutes until the gravy thickens to a glossy coating.

Stir in frozen green peas and remove skillet from heat. Season meat filling with salt and black pepper to taste.

Spread the meat filling evenly in a 9x13-inch ceramic baking dish (or leave in oven-safe cast iron skillet). Spoon mashed potatoes over the top, spreading to the edges with a spatula to seal. Use the tines of a fork to create decorative ridges across the surface.

Melt remaining 2 tablespoons butter and brush over the potato peaks. Sprinkle remaining 1/2 cup cheddar cheese over top. Bake at 400°F (200°C) for 25-30 minutes until bubbling and golden-brown. Broil for 2-3 minutes for extra crispy peaks. Rest 10 minutes before serving.