Cut green leaves and white stalks off of fennel and discard. Trim the bottom, then slice the fennel bulb into slices like you would an onion. Slice cabbage the same way. Place vegetables on a foil lined baking sheet that has been sprayed with olive oil spray. Drizzle vegetables lightly with olive oil, season with salt and pepper, and toss to distribute. Roast vegetables at 400 for 20-25 minutes or until soft and tender. Remove from oven and drizzle with balsamic vinegar. Add citrus and toss. Serve immediately.