Preparation
Prepare Basil-Butter Sauce and keep warm.
Place the rack in the top third of the oven and preheat the oven broiler.
Combine the butter, salt, pepper, and basil in a small bowl. Place the crabs on a broiler pan with the top side down and baste with the butter mixture. Broil about 4 inches from heat until reddish-brown, for about 4 to 5 minutes. Watch closely to prevent overcooking. Use tongs or spatula to turn the crabs, and baste and cook until done, about 4 to 5 more minutes.
Serve with Basil-Butter Sauce.
Basil-Butter Sauce
In a small saucepan, combine onions, wine, and pepper. Bring the mixture to a boil, and then lower the heat and let it simmer for 2 minutes.
Cut the butter into cubes. Add about 1/3 of the butter to the sauce, beating it with a whisk over very low heat. Do not allow it to boil, or it will curdle. Continue adding the butter in thirds and beating it until it is melted and heated through.
Remove the sauce from heat. Stir in the basil and parsley. Add the lemon juice and beat again. Pour the sauce over the broiled crabs.