Preheat oven to 375. Trim the ends and any brown spots off the fennel. Chop a handful of the feathery tops and set aside for garnish. Discard the rest. Cut the fennel bulb through the core into 1/2 inch slices. Combine the fennel, potatoes, and garlic in a shallow roasting pan. Drizzle with the olive oil and toss to coat. Roast for 10 minutes. Stir the vegetables and continue roasting until lightly golden and nearly tender when pierced with the tip of a knife, about 10 minutes longer.