Heat toaster oven to 400° F.
Brush one side of each tortilla with the oil. Turn 4 of the tortillas over and top with the Monterey Jack, onion, corn, jalape�os, and cilantro. Sandwich with the remaining tortillas, oiled-side up.
Working in batches, transfer the quesadillas to a foil-lined toaster-oven tray. Bake, turning once, until the cheese melts, 6 to 8 minutes total. Serve with the sour cream and salsa (if using).