In a large bowl, combine the grilled potatoes, chopped onion, mayonnaise, and pickle relish. Season with salt and pepper; stir to combine. Taste, then season with salt and pepper if desired.
Transfer to a cutting board; roughly chop.
Meanwhile, grill the seasoned onion rounds 3 to 4 minutes per side, or until lightly charred and softened.
Remove from the grill and carefully open the foil packet.
Fold the three open edges inwards to completely seal the packet. Grill 20 to 22 minutes, or until the water has cooked off and the potatoes are tender when pierced with a fork.
Add 2 tablespoons of water. Drizzle with olive oil and season with salt and pepper; toss to combine.
Place the potato rounds on one side of the foil.
Place a large piece of foil on a work surface.
Place in a bowl; drizzle with olive oil and season with salt and pepper; turn to coat.
Peel the onion and cut crosswise into 1/2-inch rounds, keeping the layers intact.
Cut off and discard the stems of the peppers. Halve lengthwise; remove the ribs and seeds.
Cut the potatoes into 1/2-inch rounds.
Fold the foil in half over the potatoes.
Preheat your grill to maintain a temperature of 450-500°F. Carefully oil the grill grates. Wash and dry the fresh produce.