In a large nonstick pan, cook red pepper, yellow pepper, onion, corn and salt to taste in butter over medium to medium-high heat for about 6-8 minutes, or until just tender. Remove vegetables from pan.
Take one tortilla . Spread 1 tablespoon Ketchup on it. Sprinkle 1/4 cup of cheese evenly over half of the tortilla. Top it with a layer of spinach leaves and further with a layer of the vegetable mixture. Sprinkle some more cheese on the vegetables, and fold the other half of the tortilla over it.
Coat the grill or pan with cooking spray.Cook until golden on both sides, for approximately 2 to 3 minutes per side. Remove quesadilla from grill or pan, and repeat with remaining ingredients. Cut each quesadilla into 4 triangles with a pizza cutter. Serve hot.