While it is preheating, clean the cauliflower. Remove the green leaves and break it into uniform sized pieces. Also remove the skin from the parsnips and slice into large chunks. You want both vegetables to be about the same size so they cook uniformly. Put the vegetables on a cookie sheet, sprinkle with olive oil, salt and pepper. Into the oven they go. Roast for 10 minutes, then remove, flip the vegetables over and return them to the oven for another 10 minutes.