Chicken Burgers

In a large skillet, heat 1 tablespoon oil over medium. Add onion, bell pepper, and celery; season with salt and pepper. Cook, stirring occasionally, until tender, 8 to 10 minutes. Transfer to a medium bowl; cool to room temperature (reserve skillet).

Add chicken to vegetable mixture in bowl; season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Mix to combine. Shape mixture into four 4-inch-wide patties.

In reserved skillet, heat remaining tablespoon oil over medium; cook patties until browned and cooked through, 6 to 8 minutes per side. Place chicken burgers on buns, and top with lettuce, tomato, and mayonnaise.