Preheat the oven to 180C/350F.
In a large skillet, melt butter and saute onions, carrots and celery. Cook for 10 minutes.
2 tablespoon butter, 1 medium white onion, 1 carrot, 1 celery stalk
Mix in beef or beef and pork mixture and start breaking it up with a wooden spoon. Cook for approximately 10 minutes, drain excess fat and season with salt, pepper and worcestershire sauce. Mix in flour and tomato paste, then pour beef stock and add bay leaves and thyme.
500 g 1 pound ground beef (or beef+pork mixed), 3 tablespoon flour, 400 ml 14 oz beef stock, 2 bay leaves, 1 tablespoon worcestershire sauce, 3 tablespoon tomato paste, 2-3 thyme sprigs
Cook for 10 more minutes, until the sauce is thick and bubbly. Taste and adjust seasoning if needed.
Transfer beef filling into a casserole pan and spread out evenly. Top with potato slices and bake covered for 1 hour.
500 g 1 pound potatoes, washed and sliced thinly
Remove the foil and cook for 30 minutes at 200C/400F or until potatoes are golden and fully cooked.