Prep the spinach: Thaw the frozen spinach and squeeze it as dry as you can in a clean kitchen towel or your hands. You want it to feel dry and crumbly, not wet, or the patties will be too loose to hold together.

Mix the patties: In a large bowl, combine the ground chicken, squeezed spinach, cheddar, panko, egg, garlic, onion powder, salt, and pepper. Mix gently with your hands just until everything is evenly combined. Overmixing makes the burgers dense.

Form and chill: Divide the mixture into 4 equal portions and shape into patties about 3/4 inch thick, pressing a shallow dimple in the center of each so they cook flat instead of puffing up. Chill for 10 minutes to help them firm up.

Cook the burgers: Heat the olive oil in a large skillet over medium heat. Add the patties and cook for 5-6 minutes per side, flipping once, until deeply golden and cooked through. Ground chicken should reach 165°F on a meat thermometer in the center.

Rest and serve: Let the patties rest for a couple of minutes off the heat, then serve on toasted whole wheat buns with your favorite toppings.