Heat the vegetable oil in a large skillet over medium heat.
Add the diced steak to the skillet, spreading it out in an even layer. Sprinkle the taco seasoning over the steak. Sauté the steak for 1-2 minutes per side until it’s lightly browned.
Add the diced red bell pepper and onions to the skillet with the steak. Sauté for 5-7 minutes, until the steak is cooked through and the vegetables are tender.
Stir in the salsa, tossing the steak and vegetables to coat them evenly. Cook for an additional 2-3 minutes to allow the flavors to combine, then remove the skillet from the heat.
Preheat a griddle over medium-high heat or use a cast iron skillet.
Place a tortilla on the griddle or skillet. Sprinkle a small amount of shredded cheddar and Monterey Jack cheese on one half of the tortilla. Layer some of the steak and vegetable mixture over the cheese, then sprinkle more cheese on top. Fold the tortilla in half to form a half-moon shape.
Cook the quesadilla for 1-2 minutes on each side, until the tortilla is golden brown and crispy, and the cheese is melted. Watch closely as the quesadilla can brown quickly.
Transfer the quesadilla to a cutting board. Slice it into wedges and serve immediately.