Peel and cube the potatoes, then place them in a large pot of salted water. Bring to a boil and cook until tender, about 15-20 minutes. Drain well.

While potatoes cook, dice the onions and carrots, and mince the garlic. Preheat your oven to 400°F (200°C).

In a large oven-proof skillet or dutch oven, brown the ground lamb over medium-high heat. Drain off excess fat, leaving about a tablespoon in the pan.

Add the onions and carrots to the skillet. Sauté until the onions are translucent and carrots begin to soften, about 5-7 minutes. Stir in the garlic, rosemary, and thyme for the last minute.

Stir in the tomato paste and cook for 2 minutes to deepen the flavor. Pour in the beef broth and Worcestershire sauce. Simmer for 10 minutes until the liquid has reduced slightly and thickened. Stir in the peas at the very end.

Mash the cooked potatoes with butter, heavy cream, and the egg yolk until smooth. Season generously with salt and pepper. Stir in half of the cheddar cheese.

Spread the lamb mixture in an even layer in your skillet (or transfer to a baking dish). Pipe or spread the mashed potatoes over the top. Use a fork to create ridges—these will get crispy in the oven.

Sprinkle the remaining cheddar cheese on top. Bake for 20-25 minutes, or until the topping is golden brown and the filling is bubbling. Let rest for 10 minutes before serving.