Heat a large skillet over medium heat and add a drizzle of olive oil or spray with cooking spray.
Add the diced onion and bell pepper (if using) to the skillet. Sauté for about 3 minutes, or until softened.
Add the minced garlic to the skillet and cook for an additional minute.
Add the ground beef to the skillet, breaking it up with a spatula. Cook until browned, approximately 5-7 minutes. Drain any excess fat.
Stir in the chili powder, cumin, salt, and pepper. Mix until well combined and cook for another 2 minutes.
Remove the fully cooked beef mixture from the skillet and set aside in a bowl.
Wipe the skillet clean and return to medium heat. Place one tortilla in the skillet.
On one half of the tortilla, add a portion of the beef mixture (approximately ¼ of the mixture), followed by ¼ cup of shredded cheese.
Fold the tortilla in half, pressing down gently with the spatula. Cook for about 2-3 minutes on each side, or until golden brown and the cheese is melted.
Remove the quesadilla from the skillet and place it on a cutting board. Let it cool for a minute, then cut into wedges. Repeat the process with the remaining tortillas and filling.