Bring 4 cups of water to boil in a saucepan on medium heat
Dice a potato and about 1/2 an onion into 1/2 inch pieces. Place 3/4 cup of diced potato and 1/2 cup of diced onion into boiling water on the stove. Peel a clove of garlic, and add that to the boiling water in the saucepan.
Simmer diced potatoes, onion, and garlic on medium heat for about 10 minutes, or until potatoes are just fork-tender.
Use a strainer, collander, or slotted spoon, to drain liquid off of cooked potatoes and onions. Make sure to save at least 1 1/4 cup of the cooking liquid.
Place cooked potatoes, onion, garlic, cashews, coconut milk, nutritional yeast, lemon juice, and salt into a high-speed blender. Add 1 cup of your reserved cooking liquid (or hot water) to the blender as well. (You may add 1/4 cup of cooking liquid later, if you want your sauce a little thinner.)
If sauce isn't hot enough to your liking, heat it in a saucepan with pasta or vegetables, until it is hot and bubbly.