Preheat oven to 450°.
Soak cashews in enough water to cover them. Soak for at least an hour. Drain.
Slice about ½″ off the head of garlic (the pointy end), exposing the cloves, and leaving the skin on. Put the garlic in aluminum foil and drizzle a teaspoon of olive oil on top. Cover the garlic with the aluminum foil.
Put cauliflower florets on a sheet pan. Drizzle a generous amount of olive oil on top, and sprinkle salt and pepper on top. Place covered garlic on sheet pan. Roast garlic and cauliflower for 20 - 25 minutes, or until cauliflower is starting to brown. Roast garlic another 15 - 20 minutes, or until soft and tender.
Heat a tablespoon of olive oil in a pan. Add chopped onion and cook on medium high heat until translucent.
Squeeze garlic out of skin and add to a blender. Add onion, cashews, roasted cauliflower (saving some for garnish, if desired), vegetable broth, and tamari. Blend until smooth and creamy. Pour mixture into a deep pan and bring to a boil. Reduce to a simmer, and cook for 10 minutes. Add salt and pepper to taste.