Heat olive oil in a skillet over medium heat. Add the minced garlic and chopped onion, cooking for 2-3 minutes until fragrant. Add the fresh spinach and cook until just wilted, about 2-3 minutes more. Remove from heat, let cool slightly, and squeeze out excess moisture.

In a large bowl, combine the ground chicken, cooled spinach mixture, crumbled feta, Panko breadcrumbs, egg, oregano, salt, and pepper. Gently mix with your hands until just combined. Do not overmix.

Divide the mixture into four equal portions and shape each into a patty about 3/4-inch thick. Place the patties on a plate and chill in the refrigerator for at least 15 minutes to help them firm up.

Cook the patties on a skillet, grill, or in the oven until a meat thermometer inserted into the center reads 165°F (74°C). This will take about 5-7 minutes per side on a skillet or grill.