Preheat oven to 450 degrees. Line a baking sheet with parchment paper. Chop cauliflower and onion (optional) into bite-size chunks. In a large bowl, whisk remaining ingredients together to form a thick, pancake like batter. Dump veggies into batter and stir until thoroughly coated. Place on baking sheet. Bake for 25 minutes, until golden brown and crispy. Flip half way to ensure both sides get crisp.