Mix flour, pepper and salt together. Use it to coat the beef cubes. Set aside floured beef.
In a large pot, melt the butter and add the onions. Caramelize the onions until translucent and golden.
Add beef, beef boullion cubes, onion soup mix. Brown meat, stirring every now and again.
Add the paprika powder and stir through the beef. Add just enough water to cover the meat and boil for twenty minutes before turning the heat down to simmer.
Simmer for 4 hours, or until meat is spoon-tender.
Add the 2 tbsp paprika powder, stir through.
Add the cornstarch dissolved in water while swiftly stirring the stew. When the stew has thickened, add the sour cream and stir the stew four hours or longer, until the meat can be cut with a spoon and practically melts in the mouth.
Serve hot over rice or buttered flat pasta noodles, or over pierogi.