1.Preheat oven to 200°C.
2.Put a large piece of tinfoil (twice the size of the fish) on a baking tray and drizzle with the oil. Place the salmon in the middle of the foil. Make diagonal cuts across the top side of the salmon and insert slices of garlic in the cuts. Lay lemon slices along the top, then scatter with thyme. Season with salt and pepper.
3.Tie kitchen string at intervals around the body of the fish, to hold it in shape. Bring the edges of the foil together, then carefully pour ½ cup water around the fish. Fold the foil edges over a couple of times to seal, but leave the parcel reasonably big and loose, so the steam will circulate while cooking.
4.Cook in the oven for 35–40 minutes or until tender. Check by inserting a skewer into the fish, leave it there for 30 seconds and then remove. If the skewer is hot, the fish is cooked.
5.Make the sauce. Put red capsicum, skin side up, under a hot grill for 5–6 minutes, until blistered all over. (Alternatively, put skin side down on a hot barbecue or chargrill.) Remove from heat, cover and leave to cool, then remove seeds and skin.
6.Cut into small dice. Score a cross on each tomato with a sharp knife, then blanch in boiling water for a few seconds, refresh in cold water and peel off the skins. Cut in half, remove seeds and dice the flesh.
7.Put capsicum and tomatoes in a pot with the lemon juice, extra virgin olive oil, basil and spring onions and gently warm. Season to taste with salt and pepper.
8.To serve, pour sauce onto a large platter. Gently remove foil from salmon, cut the string and carefully transfer salmon to platter. Serve.