1. Preheat a grill or grill pan on medium heat. Whisk together the coconut milk, curry powder, fish sauce, salt, red pepper flakes, lime zest and lime juice in a medium bowl.
2. Place 2 large pieces of heavy-duty foil from an 18-inch box on top of each other and put the mussels, garlic and onions on top. Fold the foil pieces upward to form a bowl or packet and pour the coconut milk mixture over the mussels. Close the packet, creating a seamless seal and leaving some space between the mussels and the top of the packet. Place on the grill and cook for 10 minutes, shaking the packet once.
3. While the mussels are cooking, brush the bread with the olive oil. Place a sheet of heavy-duty foil on the grill next to the mussels and add the bread slices. Grill until the slices begin to char, flipping halfway through, 2 to 3 minutes.
4. Remove the mussels from the grill and open the packet, being careful of steam. Pour the mussels and the liquid into a serving bowl, sprinkle with the cilantro and scallions and serve with the grilled bread.