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.

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.

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.

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.