Recipe(tried): Beef Stroganoff

Shelly,
This comes from my, "Russian Regional Recipes" cookbook, and hails from Moscow and St. Petersburg. It is an especially nice one, and a little different from the usual beef stroganoff recipe, which has tomato paste in it.
Enjoy, Angel.

Beef Stroganoff

Serves...6.

Ingredients~~
1 tablespoon dry mustard powder
1 talespoon sugar
few drops of water
90ml / 6 tablespoons sunflower oil
3 large onions, sliced
450g / 1lb fresh mushrooms, sliced
1.25kg / 2 1/2 lb fresh beef fillet, cut into 1cm / 1/2" wide strips
salt and freshly ground black pepper
600ml / 20fl ozs sour cream
6 sprigs of fresh parsley, finely chopped
deep-fried straw potatoes (optional)

Method~~
Combine mustard and sugar in a bowl with a little water to make a paste. Leave to one side for a while so the flavours can "marry".
Heat the oil in a large, heavy-based, shallow casserole. When just crackling add onions, reduce the heat to low and stir. Gently soften the onions, covered, for about 15 minutes, stirring occasionally.
Add the mushrooms, and cook on low heat for a further 10 minutes. Remove from the heat, drain the mixture, and set aside in a bowl.
Heat the remaining oil in the casserole. Drop in half of the meat, stirring with a wooden spoon, and turning to ensure even browning. Transfer to the bowl of vegetables with a slotted spoon; saute the remaining meat.
When all the meal is browned, return the meat and vegetables to the casserole, together with the mustard paste. Stir to combine.
Season to taste with salt and the freshly ground black pepper.
Add the sour cream, a little at a time, stirring continuously.
Cover the casserole, heat through gently for about 5 minutes, and serve.
Top each serving with a sprinkling of fresh, chopped parsley, and the straw patatoes, if desired.