Preheat your oven to 375°F.
Using a sharp knife, create a pocket in the chicken breast by slicing horizontally, being careful not to cut all the way through.
In a small bowl, mix the fresh spinach with crumbled feta cheese. Season lightly with salt and pepper.
Stuff the chicken pocket with the spinach and feta mixture, securing with toothpicks if necessary.
Season the outside of the chicken breast with salt and pepper.
Heat a small oven-safe skillet over medium-high heat and add the chicken breast. Sear each side for about 2 minutes until lightly browned.
Transfer the skillet to the oven and bake for 20-25 minutes, or until the chicken is fully cooked.
Meanwhile, steam the chopped cauliflower until tender, about 8-10 minutes.
In a blender or food processor, combine the steamed cauliflower, garlic, olive oil, unsweetened almond milk, salt, and pepper. Blend until smooth and creamy.
Plate the sliced stuffed chicken with a generous serving of creamy garlic mashed cauliflower on the side and enjoy.