Preheat the oven to 425 °F. Line the baking tray with parchment paper if desired.
Chop the cauliflower and onion into large pieces. Toss with olive or neutral-flavored oil.
Place the garlic in foil. Drizzle with olive oil. Close the foil to create a packet.
Roast the cauliflower, onion, and garlic on a sheet pan in the oven for 25 minutes.
Place the cauliflower, onion, and garlic in a high-speed blender or food processor. Add the white wine vinegar, white miso paste, olive oil, soaked cashews, and salt. Blend until smooth. Add water as needed to get the desired consistency. Season with salt and pepper to taste.