Prep the vegetables: Dice the zucchini and bell pepper into uniform 1/2-inch cubes. Finely dice the red onion. If using fresh corn, cut kernels off the cob. If using frozen, make sure kernels are fully thawed and blotted dry with a paper towel.

Sauté the vegetables: Heat 1 teaspoon olive oil in a large non-stick skillet over medium-high heat. Once the oil shimmers, add the zucchini, bell pepper, corn, and red onion. Spread into an even layer. Cook without stirring for 3 minutes to get a light char. Then stir and cook for another 2 minutes, until vegetables are tender but not mushy. Sprinkle with salt, pepper, and optional cumin. Transfer to a bowl and let cool for 2 minutes.

Assemble the quesadilla: Wipe the skillet clean. Place one tortilla on a cutting board. Sprinkle half the cheese evenly over the entire tortilla. Spread half the vegetable mixture over one half of the tortilla. Sprinkle with half the cilantro. Fold the empty half over the filling. Repeat for the second quesadilla.

Cook the quesadilla: Return the skillet to medium heat. Place one quesadilla in the dry skillet (no oil). Cook for 2-3 minutes, until the bottom is golden brown and crispy. Carefully flip with a spatula. Cook another 2-3 minutes, until the second side is golden and the cheese is fully melted. The tortilla should feel firm and sound crisp when tapped.

Rest and slice: Transfer the quesadilla to a cutting board and let it rest for 1 minute. This allows the cheese to set slightly so the filling doesn't fall out when cut. Slice into wedges. Repeat with the second quesadilla. Serve immediately.