Dice beef and season with salt & pepper, toss with a little olive oil. Heat oil in a large, deep frying pan over medium-high heat. Cook the beef, in batches, for 2 to 3 minutes or until browned. Transfer to a bowl.
Add onion to pan, sauté for 5 minutes or until softened. Add mushrooms and garlic, sauté for 3 minutes until mushrooms are softened. Add tomato paste and cook, stirring for another minute
Return beef to pan and add the paprika and beef stock, simmer gently for 20 minutes, then add the sour cream. Season with mustard, salt & pepper to taste. Sprinkle with parsley & serve with steamed rice.