Place 4 of the tortillas on a work surface. Sprinkle one-quarter of the corn, beans, 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.
To cook them on the stovetop: Heat a large, lightly greased with cooking spray a skillet or grill pan over medium heat. Add 1 quesadilla and cook for 4 to 5 minutes, then flip over and cook another 4 to 5 minutes. It's done when it begins to get brown flecks and the cheese is melted.
To bake: Preheat the oven to 375 degrees. Place the quesadillas on a large, ungreased, rimmed baking sheet (use nonstick spray oil if the tortillas are on the dry side) and bake for 5 minutes per side.
Transfer the quesadillas to a cutting board and cut each into 4 wedges. Let them rest for 1 to 2 minutes before serving.
Reheating: Quesadillas are best eaten fresh. If they need to be reheated, avoid the microwave, which makes them soggy, and the oven, which dries them out. Instead, warm the quesadilla in a nonstick skillet over medium or medium-high heat, turning it once, until the cheese softens and the tortilla crisps slightly, 3 to 4 minutes total. (Those with thick fillings will remain somewhat soft.).