Preheat oven to 400 F. Grease a 9 x 13 casserole dish.

Place cauliflower florets in a steamer basket set over a large pot. Bring water to a boil, cover and steam cauliflower until very tender, about 15-20 minutes.

Add steamed cauliflower to a large bowl. Add the sour cream, egg, garlic, salt and half the butter and half the cheese. Mash or blend with a hand mixer to a smooth consistency.

In a large safe skillet, add the remaining butter and heat over medium-high. When melted, add the lamb and cook, stirring to break up the meat. Add the Worcestershire sauce and peas.

Spread cooked lamb onto the bottom of the casserole dish. Top with cauliflower mash and sprinkle the top with cheese.

Transfer to the oven and bake 20-25 minutes to golden.