Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the thinly sliced flank steak, onion, and optional green bell pepper. Season with salt and pepper. Cook for about 5–7 minutes, until the steak is browned and the vegetables are tender.

Remove the steak and vegetable mixture from the skillet and set it aside on a plate, allowing flavors to meld.

In the same skillet, reduce heat to medium. Place one large flour tortilla in the skillet. On one half, layer a portion of the cooked mixture followed by cheese. Fold the empty half over the filling and press lightly to seal.

Cook the folded quesadilla for about 2–3 minutes on one side until golden brown. Carefully flip and cook for another 2–3 minutes until the other side is crisp and cheese is melted.

Remove the cooked quesadilla and keep warm while repeating the process with remaining tortillas and filling.

Once all quesadillas are cooked, let cool for a minute. Slice each into wedges and serve warm with optional sour cream and salsa.