Heat oven to 375° F. Spread the hazelnuts on a rimmed baking sheet and toast, tossing occasionally, until fragrant, 5 to 6 minutes; roughly chop.
Meanwhile, fill a large saucepan with 1 inch of water and fit with a steamer basket (or fill a large pot with 1/2 inch of water). Bring to a simmer. Place the broccoli in the steamer basket, cover, and steam until tender, 4 to 5 minutes. Transfer to a platter.
Meanwhile in a small saucepan, cook the oil and garlic over low heat until the garlic is just golden, 4 to 6 minutes. Transfer to a small bowl and stir in the hazelnuts, lemon juice, and 1/2 teaspoon each salt and pepper. Drizzle over the broccoli.
Make-Ahead Tip: Toast and chop the hazelnuts and store at room temperature in a resealable container or bag for up to 4 days. Cut the broccoli into florets, wrap in a damp paper towel, and refrigerate in a resealable plastic bag for up to 1 day.