Heat the oil in a skillet over medium-high heat.
When the oil is hot, add the onions and cook, stirring occasionally, for about 15 minutes. If the pan starts to get dry, reduce the heat to medium-low or add a little bit of water.
Once the onions are soft and brown, remove them from heat.
Briefly heat your cooked and sliced steak in the microwave until it’s just slightly warmer than room temperature.
Place a tortilla on a clean, flat surface. Layer half of the tortilla with 1/4 of the steak, 1/4 of the goat cheese, 1/4 of the red pepper, and 1/4 of the onions.
Fold the other half of the tortilla over the filling to create a half-moon shape.
Repeat with the remaining tortillas.
Heat a nonstick griddle over medium-high heat. When the griddle is hot, spray with cooking spray.
Immediately add two quesadillas and cook until the bottoms are nicely browned, about 3 minutes.
Flip the quesadillas, cook for another 3 minutes to brown the other side, then transfer to a cutting board.
Allow the quesadillas to rest for 1-2 minutes, then slice each one into 3 wedges. Repeat until all quesadillas are cooked and sliced.