Preheat oven to 475°.
Combine first 4 ingredients, stirring well with a whisk. Cover and chill.
Combine asparagus, onion, and garlic in a jelly roll pan coated with cooking spray. Coat vegetables with cooking spray. Bake at 475° for 10 minutes.
Spoon vegetable mixture and roasted peppers over tortillas; sprinkle evenly with cheese. Fold tortillas in half. Place a large nonstick skillet over medium heat. Add quesadillas, 2 at a time, to pan, and cook 3 minutes on each side or until toasted, pressing with a spatula. Cut each quesadilla into 4 wedges. Serve immediately with sour cream mixture.