To make the black bean spread, combine the black beans, onions, garlic, lime juice, cilantro, cumin, chili powder, salt, pepper, and 1 tablespoon water in a food processor. Process until beans are creamy and smooth, adding another tablespoon of water only if necessary. Season with additional salt and pepper if desired.
Spread a layer of the black bean mixture on top of the tortilla, top with cheese, and then another tortilla. Heat both sides on a buttered griddle or pan over medium heat for 2-3 minutes per side, until cheese is melted. Let cool for one minute before cutting into wedges. Serve with guacamole and salsa.