Brown the ground beef; drain.
In a saucepan, cover potatoes with water and boil until tender.
Add onion and carrot to the beef. Season with salt and pepper and continue cooking until veggies are almost tender.
To the meat mixture, add peas and continue cooking over low heat.
In a small pan, melt the butter and stir in the flour, cooking for 1 minutes.
Add beef stock to flour mixture and whisk quickly. Add Worcestershire sauce. Pour this mixture over meat mixture, stirring to combine.
Pour meat mixture into an ovenproof casserole dish.
Drain the potatoes, then return to the pot. In a small bowl mix the sour cream, milk, egg yolk. Add this mixture to the potatoes and mash. Spread this over the meat mixture. Sprinkle with paprika. Broil 5-10 minutes.