A true Stroganoff has very little sour cream and no mushrooms, tomato paste, or paprika. What makes it special is the spicy mustard. It's best made with beef tenderloin, but if your budget won't allow it, use sirloin.
Spread beef strips out on heavy brown paper and sprinkle evenly with salt and pepper. Toss to mix, spread out again, top with onion slices, and let stand 2 hours at room temperature. Melt 2 tablespoons butter in a large, heavy skillet over moderately low heat and blend in flour. Mix in stock and heat, stirring, 3-5 minutes until thickened. Blend in mustard and remove sauce from heat. In a second large skillet, melt remaining butter over moderately high heat, add beef and onions, and brown quickly--this will take about 10 minutes. Add browned beef but not the onion to the sauce, set over moderate heat, cover, and simmer, stirring once or twice, 15 minutes (onion can be saved for hamburgers or stew). Remove from heat, stir in sour cream, and serve. Serve with warm egg noodles. Serves 4.