Mix burger ingredients: In a large bowl, mix together the ground chicken, spinach, cheddar cheese, breadcrumbs, onion, garlic, Dijon mustard, Worcestershire sauce, salt, and pepper until well combined. Use your hands to gently incorporate all ingredients, being careful not to overmix as this can make the burgers tough.
Form patties: Shape the mixture into 4 equal patties, pressing them gently but firmly to hold their shape. Make a slight indentation in the center of each patty with your thumb to prevent the burgers from puffing up in the middle while cooking.
Cook the burgers: Heat a drizzle of olive oil in a skillet over medium heat. Once hot, add the patties and cook for 5–6 minutes per side, or until the internal temperature reaches 165°F (75°C). The burgers should be golden brown on the outside and completely cooked through.
Rest the burgers: Let the patties rest for a few minutes after cooking to retain their juices. This short resting period allows the proteins to relax and the juices to redistribute throughout the burger, resulting in a more moist and flavorful patty.
Assemble and serve: Toast the burger buns lightly, then assemble each burger with your favorite toppings such as lettuce, tomato, or an extra layer of melty cheddar. Serve warm and enjoy the balance of cheesy, garlicky flavor with a touch of spinach freshness.