Preheat the oven to 220C/Gas 200C/Gas 7. Put the sliced onion, pepper and new potatoes on a large baking tray and drizzle with 1tsp of the oil. Season with a little salt and lots of freshly ground black pepper. Toss the veg together well and spread them out on the tray. Bake in the oven for 20 minutes or until the vegetables are lightly browned and beginning to soften.
While the vegetables are cooking, make the stuffing for the fish. Put the ciabatta pieces in a large non-stick frying pan and place it over a medium heat. Cook for 4-5 minutes, turning regularly until the chunks of bread are lightly toasted.
Cut the sun-dried tomatoes into thin strips and finely slice the garlic. Add the pine nuts, garlic and tomatoes to the frying pan and toss together over a low heat for 2 minutes more until the nuts are lightly toasted. Remove the pan from the heat and leave to cool for a few minutes. Add the parsley and basil leaves and toss together.
Working carefully, as some sea bass fins are very spiky, slash the fish diagonally with a knife 4 times down each side. Sprinkle with half the lemon juice, making sure plenty goes into each cut and season with black pepper. Take the tray with the vegetables out of the oven and turn them with a spatula. Push the veg into a heap down the centre of the tin and place the fish on top.
Open the fish and spoon the bread, tomato and herb filling inside. Don't worry if a few pieces fall on to the tray, but try to keep them close to the fish or they could burn. And remember that the backing tray will be hot, so watch your fingers. Close the fish and drizzle with the remaining lemon juice and the rest of the oil.
Bake for 20-25 minutes or until the fish is just cooked and the filling is hot. Check by sliding a knife into the thickest part of the fish and peering inside - the fish should look white and a little flaky rather than translucent. Serve right away - nothing else needed.