Season fish with salt and pepper. Heat the olive oil in a large skillet. Sear the tomatoes over medium heat, add the leeks, and cook together 3 minutes. Add the shallots, olives, capers, reserved tomato juice, vermouth, and stock, and cook 5 minutes more over medium heat, or until the sauce reduces slightly.
Pour the vegetable mixture into an ovenproof casserole. Place the fish on top of the bed of vegetables and roast the fish for 8 to 10 minutes. Gently remove the fish from the casserole, stir in the tarragon and butter and spoon the vegetable sauce around the fish.
Serve immediately, sprinkled with additional chopped tarragon.