Brussels Sprouts With Red Onions and Hazelnuts

Ingredients
2 pounds Brussels sprouts
1/4 cup olive oil, divided
1 small red onion, thinly sliced lengthwise
2 tablespoons balsamic vinegar
1/4 cup hazelnuts, toasted and chopped
salt and freshly ground black pepper
Serves 8
Instructions
1. Preheat oven to 475 degrees F. Line a rimmed baking sheet with parchment paper and set aside.
2. Trim outer leaves and stems from Brussels sprouts, and discard.
3. Halve Brussels sprouts. Toss with 3 tablespoons olive oil, salt and pepper, and arrange on baking sheet.
4. Roast 20 minutes, until golden brown, tossing once halfway through.
5. Meanwhile, add remaining tablespoon of oil to a small skillet over medium-low heat. Add onions and cook, tossing occasionally, until wilted and transparent, about 3 to 4 minutes.
6. Add vinegar (stand back to avoid the fumes), and stir to loosen any brown bits on bottom of pan. Cook until vinegar is reduced and the onions are glazed, about 30 seconds.
7. Add onions to Brussels sprouts, and toss well. Sprinkle hazelnuts over top and serve immediately.