Step 1Heat the oven to 450 degrees. If the fish steaks have any scales, scrape them off with a serrated knife, then rinse the steaks and pat them dry. Line a heavy roasting pan with foil to make sure they won't stick and for easy cleanup; lightly oil the foil or the pan. Set the fish steaks side by side in the pan.
Step 2Mince 1 garlic clove and put it in a small bowl. Add the lemon juice, thyme, cayenne to taste and 1 tablespoon of oil. Sprinkle the mixture on both sides of the fish steaks and rub it in. Sprinkle the fish evenly with salt and pepper to taste, then let marinate briefly while you prepare the pistou.
Step 3Coarsely chop the remaining garlic and put it in a food processor. Add the basil leaves and blend to chop the mixture finely. With the motor running, gradually add 4 tablespoons of olive oil. Scrape down the sides and process again so the mixture is well blended. Season to taste with salt and pepper. If you like, add the remaining tablespoon of olive oil and process briefly to blend. Reserve the pistou at room temperature.
Step 4Roast the fish in the oven for 5 minutes. If using fish steaks, turn them over. Bake the fish for 3 more minutes. Pierce its thickest part with the point of a thin knife and check its color inside. It should be opaque and paler pink than when raw; if you like salmon medium, its center should still be slightly deeper pink than its sides. When you check with a fork, the fish should just begin to flake but should not flake easily because it continues to cook when removed from the oven.
Step 5Serve the fish on a bed of lettuce. Surround it with cherry tomatoes and lemon wedges and garnish with basil sprigs. Spoon about 1 tablespoon of pistou on each fish steak or serve the pistou separately.