Spinach, Black Bean, and Avocado Quesadillas

First, sauté onions with spinach and taco seasoning in a pan. Once softened, add canned black beans to the mixture and warm through. In a separate bowl, mash the avocado and season it with salt and lime juice to taste, if desired. To assemble the quesadilla, spread the mashed avocado on one side of a tortilla. Top the avocado layer with the black bean and spinach mixture, then sprinkle with cheddar cheese. Place a second tortilla on top to form a sandwich. Heat a pan with a little oil over medium heat, then cook the quesadilla for a few minutes on each side until golden brown and the cheese is melted.