1 onion, coarsely chopped
4 cloves garlic, minced
2 medium zucchini, about 1 lb, halved lengthwise and thinly sliced
1 cup frozen corn kernels
1/2 cup chopped cilantro
4 (8-inch) flour tortillas
2 cups grated pepper jack cheese
Preheat oven to 400°. In a large skillet, heat 3 Tbsp oil over medium heat. Add onion and 1 tsp salt; cook, stirring occasionally, until onion is soft, about 5 minutes. Stir in garlic; cook 1 minute more.
Add zucchini and frozen corn kernels; cook, stirring occasionally, until zucchini is soft and corn is tender, about 6 minutes. Remove from heat; stir in cilantro, if using.
Brush one side of all tortillas with remaining Tbsp oil; lay 2 tortillas, oiled side down, on a baking sheet. Place half of the filling on each, and sprinkle with half the cheese. Place remaining 2 tortillas on top, oiled side up; press down gently with a spatula to seal.
Bake until cheese has melted and tortillas are golden brown, turning once, about 10 minutes. Remove from oven; let cool slightly. To serve, slice each quesadilla into wedges.