How to Make It
Light a charcoal grill or preheat a gas grill. Brush the peppers and onions with olive oil and grill over high heat until softened and charred in spots, 8 to 9 minutes. Transfer the peppers to a bowl, cover with plastic wrap and let cool. Peel the peppers and cut into thin strips.
Lightly brush the bread on both sides with olive oil and grill until toasted, 2 to 3 minutes. Transfer to a wire rack. In a small bowl, mix the mayonnaise with the pesto sauce and spread it on the bread. Preheat the broiler.
In a bowl, mix the tablespoon of olive oil with the vinegar. Add the onions and peppers, season with salt and pepper and toss to coat. Mound the peppers and onions on the toast. Cover with the cheese and broil for 1 to 2 minutes, turning the rack, until melted. Serve.