Preheat oven to 400 degrees F. Place asparagus in a 15x10x1-inch baking pan. Drizzle with oil, sprinkle with salt, and toss to coat. Roast, uncovered, for 15 to 20 minutes, until asparagus is crisp-tender, tossing once. Transfer to a serving platter.
Meanwhile, for dressing, shred enough peel from one orange to equal 1 tsp.; set aside. Juice half an orange. Peel then slice the remaining half and whole orange half in rounds. In a jar with a tight-fitting lid, combine orange peel, orange juice, garlic, mustard, fennel seeds, olive oil, and vinegar. Cover with lid and shake to combine.
Drizzle a little of the dressing on the asparagus; toss to coat. Carefully toss in orange slices. Pass any remaining dressing.