Combine the potato, bell pepper, onion, and cashews in a small saucepan and cover with water. Bring the water to boil over high heat, reduce the heat to medium, and cook until the potatoes are very tender, about 10 minutes. Drain the vegetables, reserving ¾ cups of the cooking water.
Combine potato mixture, reserved cooking water, tahini, lemon juice, nutritional yeast, arrowroot powder, and salt in a blender. Process on high until everything is smooth and creamy about 3 minutes.
May store the sauce in an airtight container in the refrigerator for up to 5 days.