Finely chop the fresh spinach into 1/4-inch ribbons and mince the garlic cloves. In a large bowl, combine the ground chicken, prepared spinach, shredded mozzarella cheese, pesto, minced garlic, salt, black pepper, panko breadcrumbs, and crushed red pepper flakes. Using your hands, gently mix until just combined—avoid overworking the mixture, as this will make the patties dense and tough. I find that mixing with a light hand keeps these burgers tender and juicy.

Divide the chicken mixture into 4 equal portions and gently shape each into a patty about 1/2 inch thick. Make a slight indent with your thumb in the center of each patty to prevent them from puffing up during cooking. While you're shaping the patties, place the burger buns on a toaster or in a dry skillet over medium heat and toast until golden brown. Set the toasted buns aside on a plate.

Heat the canola oil in a large skillet over medium-high heat until shimmering—about 1-2 minutes. Once hot, carefully place the patties in the pan and cook for 5-6 minutes without moving them, allowing the bottoms to develop a golden-brown crust. Flip the patties and cook for another 5-6 minutes until the internal temperature reaches 165°F. I like to use a meat thermometer to ensure they're cooked through without overdrying them.

Remove the patties from the heat and let them rest on a clean plate for 2-3 minutes. This short resting period allows the juices to redistribute throughout the patty, keeping them moist when you bite into them. Place each warm patty on a toasted bun and serve immediately with your desired toppings.