Preheat the oven to 180°C (350°F), gas mark 4.
In a bowl, add the cashews and pour over just-boiled water. Leave them to soak for 5 minutes.
Scrub the sweet potatoes, leaving the skins on, and trim the ends. Cut them in half lengthwise. Once the oven is heated, place the sweet potatoes on a baking tray and roast for approximately 1 hour (they are ready when a fork easily pierces the flesh). Remove from the oven and allow to cool for 5–10 minutes.
Once the sweet potatoes have cooled, scoop out the flesh into a large mixing bowl, leaving a thin layer of flesh around the skins to help them hold their shape. Place the skins back on the tray and return to the oven for another 10 minutes to crisp up.
While the skins are baking, mash the scooped-out sweet potato flesh in the bowl. Drain and rinse the black beans and sweetcorn (if using tinned), then add them to the bowl.
Deseed and finely chop the red chilli, then chop the scallions and garlic. Finely chop the coriander. Add all of these to the bowl with the black beans, sweetcorn, cumin, paprika, lime juice, salt, and black pepper. Mix everything thoroughly.
Drain and rinse the cashews, then add them to a blender along with the oat milk, garlic powder, lemon juice, and salt. Blend until smooth. If your blender struggles to achieve a smooth texture, strain the mixture through a sieve to get a creamy sauce. The thicker remains can be used as an additional topping.