Ingredients

• 1/2 cup blanched hazelnuts
• 1 large bunch fresh tarragon
• 1 3-3 1/2-pounds skinless halibut fillet (halved lengthwise if very wide)
• 4 tablespoons olive oil, divided
• Kosher salt, freshly ground pepper
• 1/2 cup (1 stick) unsalted butter
• 1/4 cup fresh lemon juice
• Lemon halves

Instructions

1. Preheat oven to 350°F. Spread hazelnuts in a single layer on a rimmed baking sheet and toast, stirring occasionally, until golden, 8-10 minutes. Coarsely chop; set aside.
2. Reduce oven temperature to 300°F. Scatter tarragon sprigs on a large rimmed baking sheet. Place halibut on top, drizzle with 2 tablespoons oil, and season with salt and pepper. Roast until halibut is just opaque in the center, 20-30 minutes, depending on thickness of fish.
3. Meanwhile, melt butter in a small saucepan over medium-high heat. Cook, stirring often, until butter foams, then browns (don't let it burn), about 5 minutes. Remove from heat; let brown butter cool slightly. Stir in lemon juice, hazelnuts, and remaining 2 tablespoons oil; season with salt and pepper.
4. Serve fish with hazelnut brown butter sauce and lemon halves.