Preheat oven to 425 F.
Remove the leaves from the cauliflower and trim the stock so the cauliflower will sit flat in the pan. Don’t cut the stock to short as you want the entire cauliflower to stay in tact.
Take a pastry brush and cover the entire cauliflower in gravy, ensuring to get all the cracks and florets on the bottom, tops and sides. Place the cauliflower into the centre of a large cast iron skillet or other rimmed baking dish. This should use up ½ of the gravy.
Place the potatoes, carrots and onions into a bowl and toss with olive oil and a little bit of salt. Place the vegetables into the cast iron skillet, surrounding the cauliflower.
Pour the vegetable broth into the skillet.
Roast the cauliflower uncovered for 30 minutes. Remove from the oven and brush ½ of the remaining gravy on top of the cauliflower (there should be ¼ of the gravy batch left). Sprinkle the chopped rosemary onto the cauliflower roast and surrounding vegetables.
Place roast back in the oven and cook for an additional 25-30 minutes. The roast is done when the gravy is nice and caramelized, the cauliflower is tender all the way through and the potatoes are soft when poked. We use a sharp knife to check.
Serve the roast with the remaining gravy to drizzle on top.
Enjoy the feast!