Cut off the top of the bulb so some of the garlic clove tops are exposed. Rinse and drain the cashews. Preheat the oven to 375 degrees. While the cashews are simmering (steps #2–3) you can roast the garlic (steps #4–5) so both the garlic and the cashews are ready around the same time.
In a sauce pan, add the cashews and 4 cups of water. Bring to a boil, then turn the heat down and simmer on low heat with the lid on for 40–45 minutes. When done, the cashews will be soft and have a smooth appearance on the outside.
Wrap the garlic bulb loosely in parchment paper and place on a baking pan or sheet pan. Roast in the oven for 35 minutes or until the clove interiors are soft and golden around the edges. We don’t want the garlic to get dark for this recipe, just golden and soft. Usually it’s just the papery garlic skin that gets too dark, but if the cloves have dark spots, cut and discard those sections. Dark garlic will give an unpleasant bitter taste.
When the garlic bulb has cooled enough to handle, squeeze out the individual garlic cloves and place in the blender cup with the cashews.
To keep the cashew particles from spattering out, wrap a paper towel around the top of the blender cup.
Start blending the cashews and roasted garlic paste with a pulsing action (turn on for a few seconds, turn off, turn on again and repeat).
Scrape down the sides of the blender cup with a flexible spatula and add 1 or 2 tablespoons of the very hot water, the salt or miso if using and continue blending. It should be a really thick paste and pretty smooth. It will get smoother and thinner in the next step.
Blend until very smooth. It should be fairly thick (forms peaks that hold). If too thick, add more of the hot water one tablespoon at a time, blending in between until the consistency is super smooth. If using lemon juice, the juice can be used in place of a tablespoon of water. The butter will have sheen and appear completely smooth when done.
The butter can be stored in the coldest part of the refrigerator in a glass or ceramic container for up to 5 days. The butter will thicken up as it gets colder and can be thinned if needed with a little warm water stirred into it with a fork.