Working at the thickest portion of the vegetable, cut the sweet potatoes crosswise into eight ½-inch-thick slices. Cut the red onion into four ¼-inch thick slices. Arrange the sweet potato and onion rounds on a rimmed baking sheet. Brush both sides with olive oil (about 2 tablespoons total) and season with salt and pepper. Roast for 15 minutes, then carefully flip and continue to roast until vegetables are tender and browned, about 10 minutes more.
Meanwhile, cook the frozen turkey burgers in a large skillet with 1 tablespoon olive oil according to package instructions. When done, transfer burgers to a plate and cover to keep warm.
Add the remaining 1 tablespoon olive oil to the skillet over moderate heat. Add the spinach and cook until wilted and tender, about 5 minutes. Season to taste with salt and pepper. Remove from the heat.
To assemble the burgers, arrange 1 slice roasted sweet potato on a plate. Top with a burger, some of the chipotle mayo, roasted onions and spinach. Top with another slice of roasted sweet potato. Repeat with the remaining ingredients to make three more stacks and serve at once.