Preheat the oven to 300°F (150°C), Gas mark 2. Heat the oil and butter in an ovenproof casserole until sizzling hot and brown the beef cubes all over in batches. Transfer to a plate with a slotted spoon while you finish the remainder.
Reduce the heat of the casserole to medium and tip in the onions and sauté for about 5 minutes until they are pale golden in color. Return the beef to the pan with the garlic, stirring to combine. Sprinkle over the flour and paprika and stir well to soak up all of the juices.
Add the tomatoes to the pan and season to taste. Bring to the simmer, then cover with a lid and transfer to the oven for 2 hours. When the 2 hours are up, stir the chopped pepper into the goulash, replace the lid and cook for another 30 minutes or until the beef is tender but still holding its shape nicely.
Just before serving, stir in the sour cream to give a marbled effect and sprinkle a little more paprika on top. Serve the casserole dish straight to the table with a separate bowl of buttered noodles so that everyone can help themselves.