Heat a small pan over medium heat. Melt the butter and cook the garlic until fragrant.
Preheat the oven to 180º C (350º F)
Toss the spinach and cook until loses a lot of water and it reduces in size. Add the cream cheese and mix until a creamy paste is formed.
Season the chicken with salt, black pepper, and garlic powder. Cut lengthwise and create the space for the filling.
Stuff the chicken breasts. and place in an oven tray --- you can use a cast iron pan.
To prepare the sauce: In a pot, over medium heat melt the butter. Add the flour and slowly the milk. Whisk continuously to avoid lumps of flour. Cook until the sauce is thick enough. Season with salt, black pepper, and nutmeg.
Cover the chicken with the sauce and sprinkle the parmesan cheese. Cook in the oven for 20 minutes.