Preheat your oven to 400°F / 205°C.
Line a rimmed baking sheet with parchment paper for easy cleanup.
In a large skillet, heat the oil over medium heat.
Add the chopped mushrooms, minced garlic, 2 teaspoons (10 ml / 0.3 fl oz) of parsley, ¼ teaspoon of salt, and the black pepper.
Sauté for about 10 minutes, until the mushrooms are tender and have released their liquid.
While the mushrooms cook, prepare the chicken.
Place your palm flat on top of a chicken breast and carefully slice it horizontally, stopping about ½ inch from the other side so it opens like a book.
Arrange the butterflied chicken breasts on your prepared baking sheet. Spoon one-quarter of the mushroom mixture into the center of each breast. If you have any extra filling, just scatter it on the pan around the chicken.
Fold the chicken breasts closed over the filling.
In a small bowl, mix the garlic powder, onion powder, the remaining 1 teaspoon (5 ml / 0.2 fl oz) of parsley, and the remaining ½ teaspoon of salt.
Sprinkle this seasoning mix evenly over the top of the chicken.
Carefully pour the milk into the baking sheet, around the chicken breasts. This will create your creamy pan sauce.
Bake for 30 to 35 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F / 74°C on a meat thermometer.
To serve, place a bed of fresh spinach on each of four plates.
Top with a stuffed chicken breast and generously drizzle everything with the delicious, creamy pan juices.
Enjoy!
Don't Dry It Out:** The key to juicy chicken is not to overcook it. Use a meat thermometer and pull the chicken from the oven as soon as it reaches an internal temperature of 165°F (74°C) in the thickest part.
