Preheat the grill to 550°F and place a cast-iron skillet on the grill to preheat. Make sure to close the grill lid.
Once the grill is hot, add the garlic, onion, a pinch of salt, and the butter to the cast iron skillet and place the skillet on grill grates. When the butter is melted, add the clams, white wine, and one of the sliced lemons.
Close the lid to the grill and let everything simmer for about 5 minutes.
Open the lid and closely watch the clams as they begin to open, gently stirring them to help them to open as evenly as possible. Remove them from the grill as they open, or remove the entire skillet if they are opening at the same time. Cove with aluminum foil to keep warm.
Place the bread straight on the grill grates to toast, about 1 minute each side.
Serve the clams hot topped with the parsley, more lemon slices, and the bread to soak up the pan juices.