In a skillet, heat the olive oil over medium heat. Add the diced onion and garlic, sautéing until the onion is translucent (about 3-4 minutes). Remove from heat and let cool slightly.
In a large mixing bowl, combine the ground chicken, sautéed onion and garlic, chopped spinach, shredded white cheddar, breadcrumbs, salt, black pepper, paprika, and red pepper flakes (if using). Mix until well integrated, being careful not to overwork the meat.
Preheat your grill or stovetop grill pan over medium-high heat.
Divide the chicken mixture into 4 equal portions and shape each into a patty. Make a small indentation in the center of each patty (this helps them cook evenly).
Grill the patties for about 5-6 minutes per side, or until the internal temperature reaches 165°F (75°C) and the outside is golden brown.
Toast the burger buns on the grill for the last minute of cooking, until lightly crispy.
Assemble the burgers by placing a patty on each bun, topping with fresh lettuce and slices of tomato. Serve immediately.
Notes
Optional red pepper flakes can be added for extra heat.