Preheat your oven to 425 degrees.
Remove the fronds from the fennel. You can save to use later or chop up some of the fronds and sprinkle it on top of the potatoes for more flavor.
Quarter the fennel bulbs. Pull apart the layers and set aside.
Cut the fingerling potatoes in half, length wise. If you have potatoes that are small, you can leave them whole. If you have some large ones, quarter those ones instead.
Trim the ends of the Brussels Sprouts and slice them in half.
Place the fennel and potatoes on one sheet pan and the Brussels sprouts on another sheet pan.
Drizzle both pans with olive oil. Season with kosher salt and freshly ground black pepper to taste.
Roast for 40-45 minutes til the potatoes are cooked through (you can easily pierce with a fork), checking after 20 minutes on the Brussels sprouts.
Combine everything into a large bowl and serve hot.