Combine the steak, adobo sauce, lime juice, cumin, chili powder, salt and garlic powder and set aside for the steak to marinate.
Heat the oil in a very large non-stick frying pan or skillet large enough to fit a tortilla over a medium heat. Cook the red onion and bell pepper until soft and slightly charred before removing from the pan and setting aside.
Turn the heat up to high and add the steak, leaving any excess marinade in the bowl. Cook for a minute or two until the steak has cooked and started to color. Again, remove from the pan and leave to drain any excess moisture away on a plate lined with kitchen paper.
To assemble the quesadillas, divide the vegetable mixture between half of each tortilla, before sprinkling over half the cheese, being sure to take it right to the edges. Top with the steak, then the remaining cheese. Season with a little salt, before folding over the tops of the tortillas to seal the quesadillas.
Wipe out the pan with a piece of kitchen paper and return it to a medium heat. Cook the quesadillas until golden on each side and the cheese inside is melted, flipping carefully with the help of a plastic spatula.
Keep the quesadillas warm while you cook the batch, cutting them in half just before serving with sour cream, salsa and guacamole.