Place chicken, onions, garlic, salt, pepper and flour in a resealable bag or in a bowl. Stir in the lime juice. Allow to sit for a few minutes to marinate.
Spray a non-stick frying pan with olive oil cooking spray and place over medium heat. Pour in the entire contents of the bag/bowl and cook until the onion has softened (there is no need to brown). Add additional lime juice to taste. Serve over spinach.