Preheat oven to 400°F. Wrap beets in foil. Place on baking sheet. Roast beets until tender, about 1 1/2 hours. Cool. Peel beets; cut into rounds. Mix orange peel, mustard and fennel seeds in bowl. Whisk in vinegar. Gradually whisk in oil. Season with salt and pepper. (Beets and dressing can be made 1 day ahead. Cover separately; chill. Bring dressing to room temperature before using.) Place fennel slices in bowl. Toss with enough dressing to coat. Cut peel and white pith from oranges. Cut oranges into rounds. Alternate beets and oranges along edge of platters, overlapping. Spoon fennel slices into center; sprinkle with olives. Drizzle remaining dressing over beets and oranges. Chop fronds; sprinkle over salad.