Cut the beef into pieces the size of a small walnut ( or slightly smaller).
Onions cut into rings.
Ceramic or glass dishes for baking (with lid) grease with butter. ( Do not use enamelware! I cook beef according to this recipe in a heat-resistant glass saucepan with a capacity of 2 liters. For dishes, based on 1 kg of beef, requires a capacity of 2-2.5 l). Spread in a greased bowl of meat and onions, stir, close the lid and put in a preheated oven to 180 degrees. It is possible to forget about the meat for two hours! But I still recommend an hour to look under the cover. And if some pieces of meat will not be fully immersed in the juice released, help them - a little press.
In a Cup of sour cream add a teaspoon of prepared mustard (I use Dijon, home cooked) and a teaspoon of flour. Stir.
After the allotted two hours, add to the meat sour cream with mustard and flour, mix, salt and pepper to taste, close the lid and send in the oven for another half an hour.
And in half an hour the most tender (and very tasty) beef, with stunningly tasty onion sauce, is ready.
Minimum effort for a very decent dish!
Serve with any garnish. I really like mashed potatoes, but we have rice on the menu today.