1. Heat the butter and oil in a large deep pan or saucepan. Add the onions and stir-fry until soft.
2. Add the garlic, red pepper and tomatoes and stir-fry until fragrant. Pour in the stock and bring to the boil.
3. Add the mussels and cover with a lid. Steam for 7-10 minutes or until the mussels have opened and are brightly coloured. Discard any that fail to open.