Turkey Burgers with Lemon Mayo

1. 3 tablespoons canola oil
2. 1 onion, halved and thinly sliced
3. 1/2 cup finely chopped celery
4. 1/2 cup finely chopped Granny Smith apple
5. 2 scallions, thinly sliced
6. 1 1/2 pounds lean ground turkey breast
7. 1 tablespoon minced flat-leaf parsley
8. 2 teaspoons finely grated lemon zest
9. Kosher salt and freshly ground black pepper
10. 1/3 cup reduced-fat mayonnaise
11. 1 1/2 teaspoons fresh lemon juice
12. 1/4 teaspoon chopped thyme
13. 4 whole-wheat hamburger buns, split and toasted
14. 4 iceberg lettuce leaves
15. 4 tomato slices

1. In a nonstick skillet, heat 1 tablespoon of the canola oil. Add the sliced onion and cook over moderate heat, stirring occasionally, until golden and softened, about 25 minutes. Transfer the onion to a bowl. Wipe out the skillet.
2. Heat 1 tablespoon of the canola oil in the skillet. Add the celery, apple and scallions and cook over moderate heat until softened, about 8 minutes. Transfer to a bowl and add the chipotle; let cool. Stir in the turkey, parsley, 1 teaspoon of the lemon zest, 2 teaspoons of salt and 1/4 teaspoon of pepper. Shape the mixture into four 1/2-inch-thick patties.
3. In the skillet, heat the remaining 1 tablespoon of canola oil. Add the burgers and cook over moderately high heat, turning once, until no longer pink inside, 10 minutes.
4. Meanwhile, in a small bowl, mix the mayonnaise with the remaining 1 teaspoon of lemon zest, the lemon juice and chopped thyme and season with salt and pepper. Spread the lemon mayonnaise on the top halves of the buns; set the burgers on the bottom halves and top with the caramelized onions, lettuce and tomato. Close the burgers and serve.