Chicken Burgers with Caramelized Onions

Coat the bottom of a large cast iron skillet with oil. Turn heat to high and wait until oil is hot but not smoking. Add onions and cook for 1 minute. Reduce heat to low or medium-low. Continue to cook, flipping onions every 8 minutes and adding oil if necessary to keep them moist, until dark and caramelized, 25 to 30 minutes.

While the onions are caramelizing, preheat an outdoor grill for medium-high heat and lightly oil the grate.

Combine ground chicken, bread crumbs, diced ham, egg, bell pepper, onion powder, garlic powder, salt, and pepper in a large bowl. Mix to combine, being careful not to work the meat too much. Form into 4 patties.

Grill on the preheated grill over direct heat until cooked through, 4 to 5 minutes per side, adding a slice of Swiss cheese to each in the last minute. An instant-read thermometer inserted into the center of the burgers should read at least 165 degrees F (74 degrees C).

Spread butter on the inside of the hamburger buns and grill, face-down, until toasty, about 3 to 5 seconds.

Place the patties with cheese on the bottom buns, top with caramelized onions, spread mustard on the top buns, and serve.