Place 4 tortillas on a work surface.

Sprinkle 1/4 of the corn, black beans, red onion, cheese, and chili powder evenly on each tortilla, leaving a 1/2-inch border.

Top with the remaining tortillas and press down to help them stick together.

For stovetop cooking: Heat a large skillet or grill pan lightly greased with cooking spray over medium heat.

Add 1 quesadilla and cook for 4-5 minutes, then flip over and cook for another 4-5 minutes, until browned and the cheese is melted.

For baking: Preheat the oven to 375 degrees.

Place quesadillas on a large, ungreased, rimmed baking sheet (use nonstick spray if tortillas are dry) and bake for 5 minutes per side.

Transfer quesadillas to a cutting board and cut each into 4 wedges.

Let them rest for 1-2 minutes before serving.

For reheating: Warm quesadilla in a nonstick skillet over medium heat, turning once, until cheese softens and tortilla crisps slightly (3-4 minutes total).