Trim outer leaves from fennel; reserve 1 tb feathery leaves. Cut fennel bulb in half crosswise & then lengthwise into thin slices. Combine fennel bulb, reserved leaves, orange rind, salt, pepper, fennel seeds, orange juice & oil in a medium bowl, tossing gently. Cover & chill. Drain, reserving liquid.
Spoon fennel mixture into center of a round serving platter. Arrange oranges & grapefruit around fennel mixture; top with onion. Place radicchio around outside of platter, tucking leaves under orange & grapefruit slices. Drizzle with reserved orange juice mixture.