Preheat the oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.

Slice each chicken breast in half horizontally to make 4 thin cutlets. Place them between sheets of plastic wrap and pound to an even ¼-inch thickness. Season both sides with salt and pepper.

Heat the olive oil in a large skillet over medium heat. Add the onion and cook for 5 minutes, until softened. Add the spinach and cook for about 5 minutes more, stirring, until wilted and the liquid has cooked off.

Transfer the spinach mixture to a bowl, let it cool for a few minutes and press out any excess liquid. Stir in the feta cheese until well combined.

Divide the filling among the chicken cutlets, spooning it onto the center of each and leaving a ½-inch border around the edges.

Roll each cutlet up around the filling from the narrow end, tucking in the sides, and secure the seam with toothpicks.

Roll each chicken breast in the bread crumbs, pressing gently, until well coated.

Place the chicken seam side down in the baking dish and pour the melted butter evenly over the top.

Bake uncovered for 30 to 35 minutes, until the coating is golden and the chicken reaches 165°F (74°C) in the thickest part. Let rest for 5 minutes, remove the toothpicks and serve.