Rinse the fish and pat dry. Using a sharp knife, score across each fish down to the bone at 2.5 cm (1 inch) intervals on both sides. This allows the seasoning to penetrate and the fish’s natural juices to release during cooking.
Preheat the oven to 200°C / 400°F / gas mark 6. In a mixing bowl, combine the halved cherry tomatoes, sliced garlic, basil leaves, spring onions, balsamic vinegar, a pinch of sea salt and black pepper, along with the finely grated zest and juice of 1 lemon. Drizzle in a couple of generous tablespoons of extra virgin olive oil and toss well to combine. Spread this mixture evenly over the base of a large roasting tray that will comfortably fit both fish (or divide between two smaller trays). Arrange the fish on top, head-to-tail.
In the same bowl used for the tomatoes, mix together the chopped olives, parsley, and the zest and juice of the second lemon. Loosen with a little extra virgin olive oil, then spoon an equal amount of this mixture over the centre of each fish. Bake in the preheated oven for 12 to 15 minutes, depending on the size of the fish. To test for doneness, gently insert the tip of a knife into the thickest part of the fish; the flesh should easily flake away from the bone.
Remove the tray from the oven and allow the fish to rest for 3 to 4 minutes before serving. Portion the fish onto 4 warmed plates, making sure to spoon plenty of the roasted tomatoes and pan juices over the top.
