Begin by placing the cleaned and deboned chicken on a cutting board. Using a meat mallet or rolling pin, carefully pound the chicken to an even thickness to ensure uniform cooking.
In a mixing bowl, combine the finely chopped onion, washed and chopped spinach, dill, mint, parsley, and fresh green onion. Crumble the feta cheese into the mixture, and season with salt and pepper. Mix everything well.
Lay the pounded chicken flat on the cutting board, skin side down. Spread the filling evenly over the chicken, leaving a small border around the edges. Carefully roll the chicken tightly from one end to the other, keeping the filling inside.
Use kitchen twine to tie the roulade at several intervals to keep it secure during cooking, maintaining its shape and ensuring even cooking.
Heat a drizzle of olive oil in an oven-safe baking dish over medium-high heat. Add the roulade and sear on all sides until golden brown to lock in juices and add flavor.
Preheat the oven to 180°C (350°F). Transfer the seared roulade to the oven and bake for 25-30 minutes, or until the internal temperature reaches 75°C (165°F).
Remove roulade from the oven and let it rest for about 10 minutes to allow juices to redistribute. Slice into thick rounds and serve.