Meanwhile, make the sauce: Chop the onions and the garlic and let them sizzle on the pan with some butter, olive oil, the bay leaves, and bacon. Add the tomato passata and the chillies and bring to a fast simmer. Add the booze if you're using any and the stock, and simmer for 10-20 minutes. Mix the milk and flour with a fork and add it to the sauce to thicken it. Remove the bay leaves and blend with a stick blender until the sauce is smooth.