Add the onion, potatoes and carrots to a dutch oven or large soup pan and cover with 5 to 6 cups of water. Bring to a boil, then cover and reduce heat to simmer for 15 to 20 minutes, until the veggies are completely soft when stuck with a fork.
In your blender, add the cooked vegetables, 1 cup of the cooking water, the softened cashews, salt, garlic, Dijon mustard, black pepper and nutritional yeast. Allow any steam to escape, then put the blender lid on and blend until completely smooth.
The sauce can be made ahead and stored in the refrigerator up to 3 days. To reheat it, just pop it into the microwave or a saucepan and warm slowly to your desired temp.