Remove mussel meat from shells and dice. Mix mussels, shrimp, basil, and bell pepper along with the marinade ingredients in a medium bowl. Divide the filling between 12 mussel shells. Set stuffed shells into a large baking sheet in a single layer.
Combine the breadcrumbs and cheese together in a small bowl. Sprinkle on top of mussel meat mixture. Drizzle some of the melted butter over each mussel.
Bake at 220°C / 425°F preheated oven for 15 minutes or until golden and crisp on top and shrimp is cooked through.