Heat the olive oil in a large skillet over medium-high heat. Add the onion and bell pepper and cook for 7-10 minutes until fork tender.
Stir in the black beans, corn, and taco seasoning. Cook for 5 minutes, stirring often. Remove from the heat and stir in the lime juice. Set aside.
Working in batches of 2, melt 1 tablespoon of butter in a large skillet over medium-high heat.
Layer half of each tortilla with ⅓ cup shredded cheese, ½ cup black bean mixture, and another ⅓ cup shredded cheese. Fold it over and cook the quesadillas in the skillet for 1 to 2 minutes per side until the cheese has fully melted and the quesadillas are golden brown. Repeat with the remaining quesadillas and butter.
Serve with guacamole, sour cream, and your favorite salsa.
Notes
Choose the right tortillas. Opt for large 10-inch flour tortillas (sometimes called "burrito size") for flexibility and ease of folding. If you're aiming for a more authentic taste or a gluten-free option, corn tortillas are a great alternative. However keep in mind they're often smaller, so you'll need to adjust the amount of filling and cheese accordingly.
Don't skip the butter. This helps toast up the outside of the tortilla and get it crispy and golden brown. It makes a huge difference in the taste!
Use a good melting cheese. Choose a cheese that melts well, such as Chihuahua, Monterey Jack, Cheddar, or Oaxaca. These cheeses will give your quesadillas the perfect gooey texture.