Heat the oven to 400 F / 200 C. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment paper.
Grate the red onion on a box grater, then squeeze it in your hand over the sink to remove extra liquid.
In a large mixing bowl, combine the ground chicken, feta, grated onion, garlic, dill, mint, oregano, egg, breadcrumbs, lemon zest, salt, and pepper.
Mix with your hands just until everything is combined and no dry streaks of breadcrumb remain. Stop as soon as it holds together, overmixing makes the meatballs dense.
Scoop the mixture into 30 g portions with a small ice cream scoop, about 16 meatballs total, and roll each one gently between your palms.
Place the meatballs on the sheet pan at least 1 inch apart so they brown instead of steam. Brush the tops lightly with the olive oil.
Bake for 18 to 20 minutes, until the tops are golden and an instant-read thermometer inserted into the center reads 165 F / 74 C.
Let the meatballs rest on the pan for 5 minutes before serving. Serve warm with lemon wedges, tzatziki, or over a Greek salad.