Wash and chop fresh spinach leaves finely.
Combine chicken, spinach, and cheese in bowl.
Add minced onion and garlic to mixture.
Crack egg into the chicken mixture.
Sprinkle breadcrumbs, salt, and pepper evenly.
Mix all ingredients gently with hands.
Shape mixture into four equal patties.
Heat grill or skillet to medium-high.
Cook patties for 6 minutes per side.
Check internal temperature reaches 165°F completely.
Toast buns lightly if desired.
Assemble burgers with your favorite toppings.