Prep filling. If using chicken, toss it with taco seasoning plus a pinch of salt and pepper.
Heat the pan. Warm a large skillet over medium heat. Add a small amount of butter or oil.
Assemble. Place 1 tortilla in the skillet. Sprinkle an even layer of cheese over half the tortilla. Add chicken and any veggies. Top with a little more cheese, then fold the tortilla over.
Cook. Cook 2 to 3 minutes per side until golden and the cheese is fully melted. Press gently with a spatula as it cooks.
Repeat. Transfer to a board and repeat with remaining tortillas, adding more butter/oil as needed.
Slice and serve. Rest 1 minute, slice into wedges, and serve with salsa, sour cream, and guacamole.