Lay the foil pieces out on the counter. Place one piece of white fish in the center of each piece of foil. Add a pinch of salt to the fish and a pinch of black pepper. Divide the clams or mussels and shrimp between each packet, laying them around the white fish and overlapping onto the fish a bit.
Make a rectangular tray of the foil around the seafood by folding up the sides. Divide the sauce evenly between each packet, being sure to divide the shallots and garlic in the sauce. Divide the other ingredients between each packet: the tomato halves, butter pieces, parsley, and remaining garlic. Add a pinch of salt to each.
Bring the two long ends of the foil toward the top, making a tent shape. Fold one edge over the other and then fold one or two more times to seal the top. Bring the end together to meet and pinch them together. The foil will look like a short box, all sealed, and about 3-4 inches tall.
Heat the grill to 375-400 degrees F. Set the foil packets directly on the grill. Rotate the packets after 7-8 minutes. Cook for another 5-8 minutes or until the clams have opened and the fish is flaking. Be careful when checking the pouches as you don’t want to get a steam burn.
Serve each packet on a plate or in a bowl, (a bowl works better) or pour the contents including the broth, into a bowl. Discard any clams that did not open. Serve with rustic bread to soak up the sauce.