To prepare fennel, cut off and discard upper stalks. Remove any wilted outer layers and cut a thin slice from each fennel base. Wash fennel and cut each bulb in half lengthwise. Cut halves lengthwise into 1-inch wedges. Place fennel and onion in a shallow roasting pan. Drizzle with olive oil; sprinkle with fennel seeds, salt, and pepper. Stir to coat.
Roast, uncovered, in a 400 degree F oven for 35 to 40 minutes or until vegetables are light brown and tender, stirring twice.