Preheat oven to 450 degrees F. Place cauliflower in a shallow baking pan. Drizzle with oil and sprinkle with pepper; toss to coat. Roast, uncovered, for 15 to 20 minutes or just until cauliflower is tender and lightly browned, stirring twice.
Meanwhile, cook fettuccine according to package directions, adding carrots for the last 2 minutes of cooking. Before draining, remove 1/4 cup of the pasta cooking water and set aside. Drain pasta mixture and transfer to a large bowl. Add roasted cauliflower and green onions; toss to combine.
In a small bowl combine the reserved pasta cooking water, mustard, honey, and salt. Add mustard mixture to pasta mixture; toss to coat. Sprinkle individual servings with cilantro and peanuts.