Preheat the oven to 375ºF.

Prepare the spice rub by mixing garlic powder, salt, and pepper in a small bowl. Set aside.

Slice each chicken breast in half lengthwise, leaving them connected like a flap.

Season the chicken with the spice rub, including inside the flap. Set aside.

In a skillet, sauté onion in 1 tablespoon of olive oil for 4-5 minutes. Add spinach, cover, and cook for 1-2 minutes until wilted. Transfer to a bowl to cool.

Mix cream cheese with cooled spinach. Divide into fourths.

Stuff each chicken breast with spinach mixture, securing with toothpicks.

Sear stuffed chicken in an oven-safe skillet with olive oil for 2-3 minutes on each side.

Transfer skillet to oven and bake for 20-30 minutes at 375ºF, flipping halfway.

Ensure chicken reaches 165ºF internal temperature before serving. Let rest for 10 minutes.

Pour any melted cheese from skillet over chicken before serving.