Prepare a skillet with cooking spray. Add spinach, onion, and garlic to the pan and cook at medium heat until the spinach is wilted and the onions are tender (about 3 minutes, stirring often.) Move the vegetables to a small bowl.
Remove the burrito from the packaging, and wrap it in a paper towel. Place it on a microwave-safe plate, and cook it for 60 seconds.
Unfold the burrito and using a spatula, scrape the chicken mixture off the tortilla and into the bowl with the sauteed vegetables.
Combine the vegetables and chicken. Add it back into the tortilla, and fold it in half.
Spray your skillet with cooking spray again and add your quesadilla to the pan. Keep an eye on it, and flip it after about 2 minutes on medium heat. Cook the second side for 2 minutes as well. If the quesadilla doesn't come out caramelized after two minutes, cook it incrementally for another 60 seconds on each side until you have a beautifully golden brown quesadilla.
Move it to a plate and allow it to rest 2 minutes before cutting it. Slice it into four pieces, and serve with pico