In a large nonstick skillet, heat the oil over medium heat. Cook the onion, stirring frequently, until it begins to soften, 4 to 5 minutes. Add the ground beef, cumin, and salt. Cook until the beef is no longer pink. Drain.

Reduce the heat to low and stir in the corn, black beans, and salsa. Cook for 2 to 3 minutes, stirring occasionally. Set aside.

(Freezing instructions begin here.)

In another large skillet or a griddle, melt a small amount of the butter over medium-low heat.

Place a tortilla in the skillet and spread about 2 tablespoons of cheese and a heaping 1/2 cup of the beef mixture over it. Top the beef with another 2 tablespoons of cheese and then another tortilla.

Cook for 4 to 6 minutes, turning once, until the tortilla is browned and a bit firm. Before turning, be sure to add butter to the other side of the quesadilla! Repeat to make 8 quesadillas.

Serve warm and cut into triangles. Top with guacamole, salsa, or sour cream.