Spinach and Feta Chicken Burgers

Name: Spinach and Feta Chicken Burgers
Ingredients: 2 lb. ground chicken; 2 c. baby spinach, finely chopped, plus more for topping; 1/2 c. finely crumbled feta cheese (about 2 oz.); 1/2 c. panko; 1/4 c. plain Greek yogurt; 2 garlic cloves, finely chopped; 1 large egg yolk; 2 tsp. kosher salt; 1 tsp. dried dill; 1 tsp. dried oregano; 1/4 tsp. black pepper; Vegetable oil, for brushing; 1 red onion, cut into 6 thick slices; 3/4 c. tzatziki; 6 brioche buns, split and toasted; 1 plum tomato, sliced; 1/2 c. sliced pepperoncini; Potato chips, for serving