Preheat oven to 400°F (200°C).
Boil potatoes until tender, about 15 minutes.
Heat oil in large pan over medium heat.
Cook onions and carrots until softened, 5 minutes.
Add garlic and cook for 1 minute more.
Add ground beef and brown thoroughly, 8 minutes.
Stir in tomato paste and cook 2 minutes.
Pour in stock and Worcestershire sauce gradually.
Simmer mixture until thickened, about 10 minutes.
Season with salt and pepper to taste.
Drain potatoes and mash with butter, milk.
Beat in egg yolk until smooth and creamy.
Transfer meat mixture to baking dish evenly.
Spread mashed potatoes over meat layer carefully.
Bake for 25-30 minutes until golden brown.