In a large mixing bowl, combine the ground chicken, chopped spinach, white cheddar cheese, breadcrumbs, Parmesan cheese, egg, minced garlic, onion powder, salt, and black pepper. Mix until well combined, but do not overmix.
Divide the mixture into four equal portions and shape each portion into a patty about 1/2 inch thick.
Heat olive oil in a large skillet over medium heat. Once hot, add the patties to the skillet. Cook for about 5-6 minutes on each side, or until the internal temperature reaches 165°F and the patties are golden brown.
While the patties are cooking, toast the hamburger buns in a toaster or on a separate skillet until golden brown.
Assemble the burgers by placing a chicken patty on each bun and topping with lettuce, tomato, and any other desired toppings. Serve immediately.