To make quesadillas: Preheat oven to 500 degrees F.
In a large shallow baking pan, toss asparagus with 1 1/2 teaspoons oil and salt and pepper, to taste, until coated well.
In a large bowl toss onion with 1 1/2 teaspoons oil until coated well and arrange them around asparagus. Roast vegetables in lower third of oven, shaking pan occasionally until tender and lightly browned, about 10 minutes.
Grease a large baking sheet. On sheet, arrange 4 tortillas in one layer and divide vegetables, Pepper Jack, and cilantro among them. Cover quesadillas with remaining 4 tortillas. Brush top tortillas with remaining 2 tablespoons oil and broil quesadillas about 3 inches from heat until golden brown, about 2 minutes. With a metal spatula, turn quesadillas over, and broil until golden brown, about 2 minutes.
Make chipotle lime sour cream dip while quesadillas broil.
In a small bowl, stir chile and lime juice into sour cream until combined well. Dip may be made 2 days ahead and chilled covered. Makes about 1 cup.
Cut quesadillas into wedges and serve with dip.