Warm the olive oil over medium heat until it shimmers.
Add onion or scallion and cook until soft, about 4-5 minutes.
Add the chard and cook another 2 minutes until wilted.
Add the black beans and taco seasoning and stir well to combine.
Meanwhile, preheat the broiler to high or warm up the grill.
Lay out the tortillas on a cookie sheet and place one slice of cheese on a half of the tortilla.
When the chard mixture is done, place a scoop or two on top of the cheese. Place another piece of cheese on top of the chard and then fold over the tortilla in half. Repeat for each tortilla.
Place under the broiler until the top of the tortilla starts to brown after about 30 seconds to 1 minute.
Remove from the oven and flip each tortilla. If you are making this on the grill then wait until the cheese is visibly melting before flipping.
Place back under the broiler until the other side of the tortilla is browning. At this point the cheese should be melted. Remove from the oven and cut each tortilla in half.
Serve with salsa and guacamole, if desired.