Preheat the oven to 220°C.
Cut the fennel bulbs into 6 wedges each, cutting through the core to keep the wedges intact. Place on a sheet pan. Cut the potatoes in half lengthwise and place them on the pan with the fennel. Drizzle 3 tablespoons of olive oil over the fennel and potatoes and then sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper. Toss with your hands.
Place the Brussels sprouts on a separate sheet pan and drizzle with 3 tablespoons of olive oil. Sprinkle with 1 teaspoon of salt and 1/2 teaspoon of pepper and toss with your hands.
Roast the vegetables in the same oven for 35 to 40 minutes, until the potatoes are tender, tossing once while cooking.
Combine the vegetables in a large serving bowl and sprinkle with salt and pepper. Serve hot.