Cook the spaghetti according to the packet instructions.
Rinse the mussels with cold water, and pat dry.
In a pan, heat up the oil and add the butter.
Peel and chop the onion, and add it to the pan, frying it until golden.
Peel and chop the garlic, and add it to the pan together with the chopped tomatoes ad chilli flakes.
Leave to cook for 2-3 minutes until the tomatoes are mushy.
Add the mussels, prawns, calamary rings and scallops, give them a stir, then cover the pan with a lid and leave to cook for 5 minutes until the mussels are open, and the other seafood are cooked through.
Drain the pasta and add it to the pan, mixing it well.
Serve it immediately with fresh parsley, parmesan and ground black pepper.