Timing is everything in this inspired dish of fresh halibut and spot prawns atop a chorizo ragoût made with small, nutty French ratte potatoes. The potatoes can be boiled in advance. The ragoût will finish cooking as the halibut emerges from the oven and the prawns hit the hot frying pan for a quick sizzle.
Preheat the oven to 400°F. Place chorizo in a large saucepan on medium heat and cook until it releases its fat, 8 to 10 minutes. Add shallots and garlic and cook until tender, 10 minutes. Stir in morels, sautéing until softened, about 10 minutes, then add 2 tsp of the thyme. Slightly cool the pan with white wine. Gently add potatoes and peas and bring the mixture to a boil. Slowly stir in butter until the ragoût thickens, then season with salt, black pepper and lemon juice to taste. Remove from the heat, cover and set aside.
Heat olive oil in a large cast-iron pan on high. Add halibut fillets all at once and sear on one side until golden brown, about 10 minutes. Do not turn the fish over. Bake halibut in the oven, in the cast-iron pan, for about 10 minutes, until the fish reaches 135°F, or fish flakes when touched with the tip of a knife. Transfer halibut to a plate and set aside.
Place the hot cast-iron pan on medium-high heat. Add prawns and a few thyme leaves and cook for 1 minute. Remove from the heat.
To serve, divide the chorizo ragoût among 4 warmed plates and sprinkle with the chives. Top each serving with a halibut fillet and 3 spot prawns, and serve immediately.