In a large bowl, combine ground chicken, grated onion, garlic, chopped dill, mint, oregano, cooked rice, 1 egg, salt, and pepper. Mix until just combined, being careful not to overwork the meat.

With damp hands, roll the mixture into 20 meatballs, approximately 1.5 inches each, and place on a plate.

Pour chicken stock into a large, deep skillet or Dutch oven and bring to a gentle simmer over medium heat.

Carefully lower the meatballs into the simmering stock. Cover and cook for 18-20 minutes until meatballs reach an internal temperature of 74°C (165°F).

While meatballs cook, prepare the avgolemono sauce by whisking 3 eggs in a medium bowl until frothy, then slowly whisk in the lemon juice and zest.

When meatballs are cooked, reduce heat to low. Slowly ladle about 1 cup of hot cooking liquid into the egg mixture while whisking constantly to temper the eggs.

Gradually pour the tempered egg mixture back into the pot while stirring gently. Cook on low heat for 3-4 minutes, stirring continuously, until sauce thickens slightly. Do not boil or the eggs will curdle.

Drizzle with olive oil, garnish with fresh dill sprigs, and serve immediately over rice or with crusty bread.