Boil potatoes in salted water until tender, about 12 minutes. Drain potatoes and pour them into a bowl. Combine sour cream, egg yolk, and chicken or vegetable broth. Add mixture into potatoes and mash until potatoes are almost smooth.
Meanwhile preheat a large skillet over medium-high heat. Add oil to hot pan with beef or lamb. Season the meat with salt and pepper. Brown and break up meat for 3 or 4 minutes. If you are using lamb and the pan is fatty, spoon away some of the drippings. Add chopped carrot and onion to the meat. Cook veggies with meat 5 minutes, stirring frequently. In a second small skillet over medium heat, cook butter and flour together for 2 minutes. Whisk in broth and Worcestershire sauce. Thicken gravy for 1 minute. Add gravy to meat and vegetables. Stir in peas.
Preheat broiler to high. Fill a small rectangular casserole with meat and vegetable mixture. Spoon (or scoop) potatoes over meat evenly. Top potatoes with cheese and broil 6 to 8 inches from the heat until potatoes are evenly browned, and serve. Makes 4 adult or 8 child-size servings.