In a large soup pot, sauté onion and garlic gently in olive oil until softened and fragrant. Add miso paste and stir to coat onions. Add cauliflower, lemon zest, cashews and water. Bring to a boil then reduce heat and simmer until cauliflower is soft, about 15-20 minutes. Add water if necessary to keep cauliflower covered. Remove from heat and let stand until cool enough to puree. Puree soup in batches in blender – be sure cover of blender is on tight as steam from soup could pop lid. Season with salt. Ladle soup into bowls, sprinkle with za’atar and drizzle with lemon oil if desired just before serving.