Preheat your oven to 350°F (175°C).
Heat olive oil in a pan. Add onion and garlic and cook over moderate heat until clear, about 5 minutes.
Add mushrooms and continue to cook until softened. Add white wine and spinach. Cook until spinach is wilted. Season to taste with salt and pepper and remove from heat; allow cooling for about 5 minutes. Add feta cheese.
Flatten the chicken thighs/breasts with a mallet between sheets of plastic film. Season the chicken breasts with salt and pepper.
Add 2 Tbsp (30 mL) of the mushroom, spinach, and feta mixture to the center of the chicken thigh/breast and roll into a ball with the seam side facing down.
Place on a parchment-lined baking tray and brush with olive oil. You may add seasoning to the olive oil of your choice (paprika, thyme, oregano, garlic).
Bake at 350°F (175°C) for 25 minutes or until an internal temperature of 170°F (77°C) has been reached. Spoon the Parmesan Cream Sauce over top.