Tip the squash into a microwaveable bowl with a splash of water, cover with clingfilm and microwave for 10 minutes until really soft. (Or steam until tender). Mash with a fork and season. Add the black beans and mash again, then stir in the jalapeño, cheddar, coriander and lime. Divide between the 4 tortillas and spread onto half of the rounds. Fold the other half over and squash down until they hold together. Heat in a dry frying pan until golden and crisp, and the filling is hot. Cut into wedges and serve with the salsa, lime wedges and more coriander.