Preheat the oven to 375°F.

In a skillet over medium heat, add olive oil. Once hot, add minced garlic and sauté for about 1 minute until fragrant.

Add the chopped mushrooms and cook for about 5-7 minutes until they release their moisture and begin to brown.

Stir in the chopped spinach and cook for an additional 2-3 minutes until wilted. Remove from heat.

In a mixing bowl, combine the cooked mushroom and spinach mixture with cream cheese, mozzarella cheese, onion powder, thyme, salt, and pepper. Mix until well combined.

Using a sharp knife, create a pocket in each chicken breast by slicing horizontally, being careful not to cut all the way through.

Stuff each chicken breast with the mushroom mixture, securing with toothpicks if necessary.

Season the outside of the chicken breasts with salt and pepper.

Place the stuffed chicken breasts in a baking dish and bake for 25-30 minutes or until the internal temperature reaches 165°F.

Let the chicken rest for 5 minutes before slicing and serving.