Skin and finely chop the onion then mix with the minced beef.
Gently fry the beef mixture until browned then stir in the flour and fry about a minute.
Stir in the stock, tomato puree and seasonings.
Dice and de-seed the green pepper then stir into the beef mixture.
Bring to the boil then reduce heat and simmer for about 15 min., stirring occasionally.
Meanwhile wash and shred the lettuce and dice the tomato finely.
Divide the mince mixture between the taco shells, garnish with lettuce and tomato then top with a little sour cream and serve.