Preheat the oven to 375 degrees F.
To prepare Brussels sprouts, remove any yellow or brown outer leaves, cut off the stems, and cut in half. In a large bowl, toss the Brussels sprouts, oil, salt, and pepper together. Once all of the Brussels sprouts are coated in oil, arrange them in a 9- x 13-inch pan.
Roast for 15 minutes. Stir Brussels sprouts around with a spatula or large spoon to even out the browning; continue to roast for another 30 minutes. Stir the Brussels sprouts again, and drizzle maple syrup over top. Roast the Brussels sprouts for about 15 minutes more, or until they are fork tender (about 45 minutes total roasting time).
Toss the roasted Brussels sprouts with the hazelnuts and serve.
To toast the hazelnuts: Preheat oven to 350 degrees F. Place the hazelnuts in one layer on a rimmed baking sheet. Roast for 10 to 12 minutes, or until lightly browned and fragrant. Keep checking on them because nuts can burn quickly.