I made this dish and did not add olive oil to the brown butter sauce. I did add extra finely chopped tarragon to the brown butter sauce. I think this sauce would go over any other fish like sole or trout as well. I served it with roasted carrots and little streamed red potatoes, smashed with spring onion and salted butter. Perfect for a small dinner gathering. I used a well worn tea towel to get the skins off the hazelnuts, in case it stained. I was also thinking you could use almonds as a substitute.
Makes 6 to 8 servings
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.
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.
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.
Serve fish with hazelnut brown butter sauce and lemon halves.