To make sauce, heat oil in a large deep frying pan. Cook onion and chilli until very soft. Add garlic, tomatoes, capers and wine. Bring to a boil. Simmer for about 30 minutes or until thickened. Season well. Stir in basil.
To make stuffing, combine all ingredients except oil in a bowl. Add enough oil to bring mixture together and give a 'crumble' texture.
Lay garfish, skin-side down, on the bench. Divide filling among garfish. Roll to enclose, securing with two toothpicks. Dust in seasoned flour.
Heat oil in a large non-stick frying pan over high heat. Cook garfish for 1 minute, turning, until golden brown (you only want to crisp and colour the fish, not cook it through). Add to tomato sauce in a single layer, along with mussels. Cook covered for about 5-8 minutes or until fish is cooked through and mussels are open.