There are divided opinions on whether to season beef Stroganoff with tomato or not. In my family's traditional recipe, as well as in many old Russian recipes, tomato is always added to the dish, and things like mustard or mushrooms are never used.

about 400 g trimmed beef strip loin/top loin or fillet
1 onion
butter

1 portion of basic brown sauce
about 2 - 3 tbsp tomato paste (or to taste)
smetana

Using a fine grater, grate the onion or chop it extremely fine. Sauté and soften the minced onion in a little butter for a few minutes, set aside.

Cut the beef in thin strips (about ½ to 1 centimetre thick and 5 to 8 centimetres long).

Melt some butter in a pan. In batches, brown the meat quickly on high heat (see the picture on right). Place the meat together with the onion in a deep oven casserole dish.

Prepare the sauce following the instructions given here, but leave out the smetana/cream.

Season the sauce with tomato paste, adding it to your taste. Add some pepper and pour the sieved sauce on top of the meat in the casserole dish (see the picture on left). The sauce should cover the meat. Mix and cover the dish with a lid.

Bake the dish at 175 - 180 °C for about 1 hour, or until the meat is tender. Just before the end of baking time, stir a dash of smetana in the sauce. Also cream or double cream may be used instead of smetana. Reheat the dish and serve.

Beef Stroganoff is often served with matchstick/straw potatoes. In Finland, sliced or cubed pickled beetroots and Russian pickled gherkins are traditional accompaniments for the dish. In my family, we often serve scalloped garlic potatoes instead of fried potatoes.
Serves 2.

Additional information:
Instead of beef fillet or strip loin/top loin you may use topside roast/inside roast or top sirloin butt, but since they are less tender, they require a longer baking time.

Recipe source: family recipe.