Preheat Oven to 375°F.
- In a medium bowl, whisk together the olive oil, salt, pepper and lemon zest. Carefully place the cauliflower head in the bowl and roll it around to coat. This will allow the seasoned oil to get throughout the head of the cauliflower.
- Place in cast iron pan and cover with foil. Roast for 30 minutes with foil. This will allow us to steam the cauliflower to help soften it before we brown it.
- In the bowl of your food processor, place all the ingredients except the oil. Pulse to make a coarse mixture, scraping down the sides as necessary. Once you have a coarse mixture, add the olive oil in a steady stream to create a bright green mixture. Adjust seasoning as desired and set aside until ready to serve.
- Remove foil from cast iron pan and continue to roast until golden brown and softened, about 15-20 minutes. A wooden skewer or thin blade of a knife should insert and easily pull out to indicate the cauliflower is fully cooked. Remove the pan from the oven and let the cauliflower rest for 5 minutes.
- To serve, carefully transfer the roasted cauliflower to a serving platter, this is most easily done with a large spatula or two. Once transferred, spoon your cashew pesto over the roasted head of cauliflower and sprinkle parmesan (if using). Slice in wedges to serve.