Add the spinach to a dry skillet over medium heat. Cook for about 2–3 minutes, stirring occasionally, just until the leaves collapse and turn deep green. Transfer the spinach to a plate and let it cool enough to handle.
Place the cooled spinach inside a clean kitchen towel or several layers of paper towel. Twist and squeeze firmly until very little liquid comes out.This step matters more than it looks. Spinach holds a surprising amount of water, and putting it directly into the meat mixture can make chicken burgers loose, watery, and harder to brown. Removing that moisture first lets the patties sear instead of steam.Finely chop the squeezed spinach.
Add the ground chicken, chopped spinach, white cheddar, panko, egg, garlic, Dijon mustard, Worcestershire sauce, salt, pepper, and onion powder to a large bowl.Mix gently with a fork or clean hands until everything is evenly distributed. Stop as soon as the mixture comes together.Pro tip: Ground chicken becomes dense when it’s aggressively mixed. Treat it more like meatball mixture than bread dough the less handling, the more tender the finished burger.
Divide the mixture into four equal portions and gently shape each into a patty about 3/4 inch thick.Press a shallow indentation into the center of each patty with your thumb. This helps the burgers cook more evenly and reduces the tendency to puff up in the middle.
Heat the olive oil in a large skillet over medium heat. Give the pan a minute or two to warm before adding the burgers.You want a steady sizzle when the patties hit the pan, but not screaming-high heat. Because cheddar is mixed throughout the burgers, moderate heat gives the chicken time to cook before any exposed cheese gets too dark.
Add the patties and cook without moving them for about 5–6 minutes. The bottoms should develop a nicely browned crust and release from the pan fairly easily.
Carefully flip each burger and cook for another 5–6 minutes, adjusting the heat if they’re browning too quickly. Check the thickest part with an instant-read thermometer. Ground chicken should reach 165°F (74°C) before serving.
Transfer the burgers to a plate and let them rest for about 3 minutes. Spread the toasted buns with mayonnaise, then add lettuce, a chicken burger, tomato, and red onion. Serve while the cheddar inside is still warm and melty.
