Cook the Beef: Heat olive oil in a large skillet over medium heat. Add chopped onion and garlic, sauté until onion becomes translucent. Add ground beef or sliced steak to the skillet. Cook until browned. Drain excess fat if necessary.

Season the Beef: Add diced bell pepper, chili powder, cumin, salt, and pepper. Cook until the bell pepper is softened and flavors are well blended.

Prepare the Quesadillas: Lay a tortilla on a flat surface. Sprinkle 1/4 cup of shredded cheese on half of the tortilla. Layer the cooked beef mixture over the cheese. Add additional cheese on top if desired, along with cilantro. Fold the tortilla over to enclose the filling.

Cook the Quesadillas: Clean the skillet and return to medium heat. Lightly oil or butter the surface. Place the folded quesadilla on the skillet. Cook for 3-4 minutes on each side until the tortilla is golden brown and the cheese has melted.

Serve: Remove from the skillet and let cool for a minute before cutting it into wedges. Serve with sour cream and salsa, if desired.