In a medium bowl, make an ice water bath. Place mussels in the water and let sit for 15 minutes. Drain the water and rinse the mussels in cold water.

Heat your grill to 450 F. Cut baguette into ½-inch-thick slices on the diagonal. Grill the slices for 1 minute on each side or until char marks appear. Season with salt and black pepper, transfer to a plate and set aside.

To make one foil pack, layer 2 12×18-inch sheets of aluminum foil on top of each other. Cut lemon in half and then into 4 wedges and place the wedges in a 6-inch circle in the center of the foil. Place ⅓ of the mussels in the middle of the lemons, and then top with 2 1-tablespoon pieces of unsalted butter and 1 minced garlic clove. Pour 1 ½ tablespoons white wine on top of the mussels. Carefully fold the foil into a pouch and pinch to seal tightly. Repeat for the remaining 2 foil packs.

Carefully grill the foil packs, making sure to not puncture the foil, for 12 minutes. Carefully open 1 of the foil packs to let the steam out. Check the mussels — they should be completely opened up. If not, reseal and grill for another 5 minutes.

When the mussels are done, spoon the mussels from each packet into individual serving bowls, pour the sauce over the top and garnish with chopped fresh parsley. Enjoy hot with the grilled crostini.