Spray a 2-quart saucepan with the cooking spray and heat over medium heat for 1 minute. Add the onion and cook until it's tender, stirring occasionally.
Stir the spinach and flour in the skillet. Gradually stir the milk in the skillet. Cook and stir until the mixture boils and thickens. Stir in the picante sauce and cheese and cook until the cheese is melted. Serve with the tortilla chips for dipping.