Put the butter in a pan over a low heat. Stir in the flour and cook, stirring all the time to make a roux. Cook until the roux is hazelnut brown in colour – make sure that it doesn’t burn. Tip in the vegetables, garlic and bay leaf, stir briefly, then pour in the tomatoes and 1½ cans of water. Crumble in the stock cube, add the hot sauce, then cover and cook for 15 mins, stirring occasionally, while you make the meatballs.