Preheat the oven to 180C/350F/Gas 4.
For the stuffed pepper, place the whole green pepper half on a baking tray and drizzle with two tablespoons of the oil. Season with salt and freshly ground black pepper and transfer to the oven to roast for 8-10 minutes, or until tender.
Heat the remaining oil and fry the onion and garlic for 3-4 minutes, or until softened. Add the chopped green pepper half, tomato, cooked rice and sage and warm through for 4-5 minutes, then stir in the parmesan. Stuff the roasted pepper half with the rice mixture.
For the mussel and basil sauce, tap the mussels against a hard surface and discard any that do not open.
Heat the oil in a pan and gently fry the onions and garlic for 5-6 minutes, or until softened. Pour in the white wine and bring to a simmer, then add the mussels and cover with a lid for 4-5 minutes, or until the mussels have opened. Discard any unopened mussels, then stir in the cream and parsley. Remove the cooked mussels from the pan and reduce the sauce down by about a third. Stir in the basil, remove the mussels from their shells and return to the sauce. Discard the shells.
Serve the stuffed pepper in a shallow dish with the mussel and basil sauce spooned around the edge.