Heat a grill for medium heat. Crank the side burner to high-heat. Seed and stem the poblano peppers and then slice into thin strips. Thinly slice the onion. Toss the peppers and onions with salt, pepper, and olive oil. Char the peppers over high heat in a large skillet, until tender and browned, about 8 minutes. Set aside.
Coat one side of each slice of bread with mayo. Set the bread mayo side down on the grill. Sprinkle the cheese on each bread slice. Top half of bread slices with pepper and onions and crush chips.
Cook the bread until golden and the cheese is melted, about 4 minutes. Fold one cheese only bread pieces onto one pepper and onion bread slice to build a sandwich. Press down on each sandwich then use a thin flat spatula to move the sandwiches off the grill Slice each sandwich in half and serve with tomatillo salsa for dipping.