Put potatoes in a large pot and add water to cover. Boil for 20 minutes. Use a steamer basket over the boiling potatoes to steam the cauliflower for 15 minutes.
In a large saute pan, heat olive oil over medium heat. Add onion and saute until softened (about 8 minutes). Add lamb and cook until browned. Add peas and cook for 2 minutes. Add flour and Worcestershire sauce and cook until the liquid is thickened (about 2 more minutes). Season with salt & pepper. Turn off the heat and set aside.
Drain the water off of the potatoes. Add the steamed cauliflower to the pot. Add the butter and half & half. Mash together until creamy (don’t worry if there are some chunky spots). Stir in 1/3 cup Parmesan cheese and season with salt.
Pour the lamb mixture into the pre-baked pie crust. Spoon the cauliflower mixture over top and spread out evenly. Use a fork to make a pattern in the cauliflower mixture. Sprinkle the top with extra parmesan.
Bake for 15 minutes. Broil for 5 minutes. Let cool for 5-10 minutes before serving.