Special equipment: 18-inch-wide heavy duty foil
Prepare a grill for medium-high heat.
Toss together the bell pepper, onion, scallions, oil, chile powder, cumin and 1/4 teaspoon salt in a very large bowl.
Tear two 18-inch pieces of 18-inch-wide heavy duty foil. Spread the vegetable mixture on one piece of foil, leaving a few inches of exposed foil as a border. Arrange the shrimp on top of the vegetables in a single layer. Squeeze the limes over the mixture. Top with the remaining piece of foil, lining up the edges. Fold the edges and crimp tightly to make a leak-proof packet.
Tear two 18-inch pieces of 18-inch-wide heavy duty foil. Arrange the tortillas, overlapping slightly, on one piece of foil. Top with the remaining piece of foil, lining up the edges. Fold the edges and crimp tightly to make another packet.
Put the shrimp packet on the grill, close the grill lid and cook 12 minutes (do not turn the packet over). The packet should puff up and look like a pillow. Slide from the grill onto a baking sheet using tongs. Set aside and let rest while you heat up the tortillas. Put the tortilla pack on the grill and cook until warm, about 2 minutes per side.
Carefully open the shrimp packet (hot steam will escape). The shrimp should be pink and cooked through and the onions and peppers just tender. Scatter the cilantro over the shrimp. Serve with the warm tortillas and salsa and avocado, if desired.
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