Hearty farro stands in for bread in this riff on the classic.
Arrange the racks in the upper and lower thirds of the oven; preheat to 450°. In a large pot, melt 3 tbsp. butter over medium heat. Add the leek. Cook, stirring occasionally, until tender, about 8 minutes; season with salt and pepper. Add the farro. Stir until coated. Add the stock, cider, and rosemary. Bring to a boil over high heat. Reduce heat to medium. Simmer, stirring occasionally, until the farro is tender, about 25 minutes. Discard the rosemary sprigs. Drain any excess liquid from the farro.
Meanwhile, on a large rimmed baking sheet, toss the mushrooms, thyme, and 1/4 cup oil; season. On a second large baking sheet, toss the cauliflower with the remaining 2 tbsp. oil; season. Transfer both baking sheets to the oven. Roast for 10 minutes. Add 2 tbsp. butter to the cauliflower, and the remaining 2 tbsp. butter to the mushrooms. Toss until coated. Rotate the baking sheets and swap them from the top to bottom racks. Roast the mushrooms and cauliflower until tender and golden in spots, about 5 minutes.
Fluff the farro with a fork. Transfer to a large bowl. Add the parsley and the roasted mushrooms and cauliflower, scraping any buttery juices from the baking sheets into the bowl. Toss until coated. Top with the oregano.