Cut meat into chunky pieces. Heat butter and oil in pan. Add garlic first, fry for 1 minute then add the chopped onion. Cook for 3 minutes, add meat and brown over high heat. Reduce heat. Add mushrooms and stir for 3 minutes. Add cornflour and stir until combined (add more flour if you prefer it a little thicker). Add milk and lemon juice (to taste), stirring constantly. Mix in tomato paste. Season with salt and pepper. Simmer for 10 minutes. Stir in sour cream and simmer for 20 minutes until meat is tender. Serve with boiled rice or noodles.