Add the water, half of the lemon zest, 2 tablespoons lemon juice, garlic, onion powder, mustard, salt, and several cracks of pepper. Blend until creamy and thick, and all cashew bits have been pulverized, scraping down the sides with a silicone spatula as you go, about 2 minutes. If using a food processor, you will need to blend for longer (the consistency will not be as silky smooth but still tastes good).