Preheat the oven to 400°F. In a large bowl, toss the butternut squash cubes with the oil, ground chipotle or cayenne pepper and a sprinkling of salt to taste. Spread out evenly on a baking tray and roast in the preheated oven for about 25 minutes until golden and tender.
To make creamy chipotle sauce, place all the ingredients in a food processor and blend together with 1/3 cup of water until smooth. Add more water, if needed, to reach the desired consistency.
Turn the oven temperature down to 350°F. Spread the tortilla chips out on the lined baking tray – enough to fill the tray generously in a thin layer. Scatter over the roasted butternut and then the black beans, spreading them out evenly across the tray. Bake in the preheated oven for about 10 minutes.
Remove the tray from the oven and top the nachos with small dollops of guacamole or mashed avocado and pico de gallo or salsa, a drizzle of chipotle tahini sauce and a scattering of spring onions/scallions. Serve immediately either directly from the tray or transfer to a large plate.