In a small bowl, stir together the butter, half the shallot, and the shredded lemon peel. Sprinkle with salt and coarsely ground pepper. Set aside.
Rinse fish; pat dry. Spread each fish open, skin side down. Rub remaining shallot and rosemary onto fish; sprinkle with additoinal salt and pepper and drizzle with lemon juice and oil.
In a charcoal grill, arrange hot coals in bottom of an uncovered grill. Place a greased 12-inch cast-iron skillet diretctly on coals to preheat. Place fish skin side down and grill for 6 to 8 minutes or until fish flakes easily with a fork. Meanwhile, place tomatoes, cut side up, in skillet next to fish; dot each with 1/4 t of butter mixture. Grill 5 minutes or until tomatoes are heated through.
Remove skillet and cut each fish in half lengthwise. In a small saucepan melt the remaining butter mixture; serve with fish and tomatoes. Makes 4 servings.