Roasted Halibut with Asparagus and Olives

Our best tip for cooking fish for a crowd? Slow-roast a single large fish fillet instead of making several small ones - it ensures that the fish is buttery and delicious. Vegetables that are best served crisp-tender are a great accompaniment because they'll be perfectly cooked by the time the fish is finished.

2 bunches asparagus, trimmed, halved lengthwise if thick, divided
6 Tbsp. unsalted butter
7 Tbsp. extra-virgin olive oil, divided
1 tsp. Aleppo-style pepper
2½ lb skinless halibut, salmon, or arctic char fillet
kosher salt
black pepper, freshly ground
2 bunches ramps or scallions, trimmed
¾ cup Castelvetrano olives, pitted, torn
¼ cup fresh chives, finely chopped
¼ cup lemon juice, freshly squeezed
2 Tbsp. fresh tarragon, coarsely chopped

To Serve:
fresh taragon leaves
boiled small waxy potatoes
lemon wedges

Preheat oven to 325°F (165°C/140°C fan, Gas Mark 3).

Thinly slice one-quarter of asparagus into coins and place in a medium bowl; set aside.

Heat butter and 2 Tbsp. oil in a small saucepan over medium. As soon as butter is melted, add Aleppo-style pepper and remove from heat.

Season fish with salt and black pepper on both sides and place on a rimmed baking sheet. Scatter ramps and remaining asparagus around fish; season with salt and black pepper. Pour butter mixture over fish and vegetables. Give vegetables a toss to evenly coat. Roast, undisturbed, until fish is just opaque in the center and asparagus is bright green and tender, 15-25 minutes, depending on the thickness of your fillets.

Toss olives, chives, lemon juice, 2 Tbsp. tarragon, and remaining 5 Tbsp. oil with reserved asparagus; season with salt and black pepper.

Transfer fish to a platter; top with ramps and asparagus. Spoon some olive mixture over, then scatter tarragon leaves over. Serve with potatoes, lemon wedges, and remaining olive mixture alongside.

Makes 6-8 servings.