Step 1

To prepare roasted onion: Preheat oven to 375°F and place a baking sheet in the oven while it preheats. Toss onion with 1 tablespoon oil, 1 tablespoon vinegar, ¼ teaspoon salt and pepper in a small bowl. Transfer the mixture to one side of an 18-inch piece of foil. Fold the foil in half and crimp the edges to seal the onion in a packet. Place the packet on the baking sheet and bake until the onion is tender, 20 to 30 minutes.

Step 2

Meanwhile, prepare burgers: Squeeze extra liquid out of spinach and place the spinach in a medium bowl. Add chickpeas and mash with a fork. Add egg, walnuts, breadcrumbs, pesto and salt and mix well. Form the mixture into six ¾-inch-thick patties.

Step 3

Heat oil in a large nonstick skillet over medium-high heat. Add the patties and cook until golden brown on the bottom, 5 to 6 minutes. Flip the patties, reduce heat to medium-low, cover and cook until the other side is golden brown and an instant-read thermometer inserted in the center registers 160°F, about 5 minutes more.

Step 4

Meanwhile, prepare sauce: Combine 1 roasted pepper, garlic, mayonnaise (and/or yogurt) and vinegar in a small food processor. Process until smooth. Slice the remaining roasted peppers into strips.

Step 5

Place the burgers on buns and top with the sauce, roasted onion and pepper strips.