In a large mixing bowl, combine the ground chicken, chopped onion, shredded zucchini (squeezed to remove excess moisture), seasoned bread crumbs, egg, salt, and black pepper. Mix thoroughly until evenly combined.
Let the mixture rest in the bowl for about 30 minutes to allow the bread crumbs to hydrate and the mixture to firm up.
Shape the mixture into patties or croquettes using your hands, about golf-ball to palm-sized depending on preference, and place them on a plate or tray.
Heat 2 tablespoons of oil in a frying pan over medium to medium-high heat until shimmering.
Cook the patties in the hot pan for about 4 minutes per side, or until golden brown and cooked through (no pink inside), turning once with a spatula or tongs.
Transfer the cooked croquettes to a plate lined with paper towel to drain briefly, then serve with a dollop of sour cream and a sprig of fresh dill.