Place the butter for the nut crust in a medium saucepan over medium-high heat. Cook for 4 minutes, until the butter is nutty smelling and golden brown. Remove from the heat and strain the butter through a fine-mesh sieve to remove any black bits.
Add the pistachios, pine nuts, sugar, lemon juice, and 1/4 teaspoon salt. Mix well, then spread out in a small parchment-lined baking sheet, about 6 inches wide and 8 inches long. Chill in the fridge for 2 to 3 hours, until the butter has set firmly, then cut the mixture into 6 equal rectangles. Return the rectangles to the fridge until ready to use.
For the wild arugula and parsley vichyssoise and the halibut fillets:
To make the vichyssoise, bring a medium saucepan of salted water to a boil and add the parsley and arugula leaves. Blanch for 30 seconds, then refresh under cold water. Strain, squeeze out the excess water, set aside to dry, then coarsely chop.
Place 1 tablespoon of the oil and the butter in a medium saucepan over medium heat. Add the shallots and sauté for 4 to 5 minutes, stirring once or twice, until soft but not colored. Add the garlic and leek and cook for another 2 to 3 minutes. Add the potatoes and cook for 5 to 6 minutes, stirring frequently, until shiny and glossy.
Pour over the chicken stock and bring to a boil over medium-high heat, then simmer for 8 to 10 minutes, until cooked by still retaining a bite. Add the blanched parsley and arugula and cook for a final minute, then remove from the heat and add the spinach, along with 1 1/2 teaspoons of salt and a good grind of black pepper. Transfer to a blender, blitz well until completely smooth, and set aside until ready to use.
Preheat the broiler to 400° F or to its highest setting. Spread the halibut fillets out on a large parchment-lined baking pan and brush them with the remaining 2 tablespoons of oil. Season with 1 1/2 teaspoons of salt in total and a good grind of black pepper and cook for 6 to 7 minutes, until the halibut is almost cooked. Remove the baking pan from under the broiler and lay a rectangle of nut butter on top of each fillet. Return to the broiler and cook for a final 2 to 3 minutes, until the crust is golden brown. Remove from the oven and squeeze over the lemon juice.
Warm the vichyssoise and spoon it into shallow wide bowls. Lay a halibut fillet on top, place the radish pieces alongside, and serve at once.
Best friends Issy and Meg have spent the last decade sitting across tables from each other, travelling the world knife and fork in hand. Photographing bowls of steaming noodles, exotic street food and some of the world's most exciting cooks, Issy makes up the photography side of the duo, while Meg records each bite in words. Considering their equal obsession for food and each other, it was inevitable that the two would eventually combine to become The Curious Pear, intent on bringing you reviews, food features and interviews with the culinary crowd, as well as pieces on their favourite eating spots from around the world. The Curious Pear are the contributing Food Editors at SUITCASE Magazine, bringing you a weekly food column at suitcasemag.com, as well as contributing for Time Out, Food52, Life & Thyme, Trends on Trends, Guest of a Guest and more!