Heat the grill to high. Put the peppers, skin side up, on a baking tray, then pop them under the hot grill for about 10 mins until the skins blister and blacken. Drop them into a bowl, cover with some cling film and leave until cool enough to handle. Peel away and discard the skins, then roughly chop the peppers.
Heat the oil over a low-ish heat in a sturdy roasting tin or in a shallow pan that has a lid and is large enough to fit both fish. Throw in the shallots and garlic and sweat briefly until soft. Stir in the tomatoes, peppers, capers, olives and half the basil leaves, then sweat for a few mins until the tomatoes soften. Pour in the wine and tomato juice. Stir and gently simmer for 10-15 mins, adding a splash of water if the sauce becomes a bit dry.
While the sauce is simmering, slash each side of the fish a few times. When the sauce is ready lay the fish on top, season if you want to and cover with a lid (cover with foil if you are using a roasting tray). Leave to cook on a low heat for 12-15 mins until the flesh feels firm when pressed.
While the fish is cooking, melt the butter in a large pan, then fry the spinach until wilted, season if you like and divide the spinach between two serving dishes. Lift the fish carefully from the pan and place on top of the spinach, neatly drizzle some of the sauce round the fish, scatter the remaining basil on top and drizzle everything with extra-virgin olive oil. Serve with some ribbon shaped pasta, like tagliatelle or pappardelle, with the remaining sauce in a bowl or side dish.