Heat 1 tbs oil in a large frying pan over medium heat. Add onion. Cook, stirring, for 5 minutes or until softened. Transfer to slow cooker.
Heat remaining 1 tbs oil in pan over medium-high heat. In batches, cook beef for 3 to 4 minutes or until browned.
Return beef to pan. Add tomato paste and paprika. Cook for 1 minute or until beef is coated. Add stock. Bring to a simmer. Transfer to slow cooker. Add capsicum and tomatoes. Season. Stir to combine. Cover. Cook on high for 4 hours (or low for 8 hours), adding mushroom in the last 30 minutes of cooking time.
Serve goulash with sour cream, fettuccine and chopped fresh parsley.
