To make the cucumber salad, stir together sesame oil, dill, rice vinegar, mirin and sesame seeds in a glass or plastic bowl; add cucumber, and toss until well coated. Cover, and refrigerate for 2 hours. Let sit at room temperature for 30 minutes before serving.
To prepare the ginger crust, stir together the ginger, spring onions and garlic in a sturdy, metal bowl; set aside. Heat 4 tablespoons groundnut oil in a small saucepan over high heat until it begins to smoke. Remove from heat, and carefully stir in the ginger mixture; set aside.
Preheat oven to 220 C / Gas 7.
Heat 2 tablespoons groundnut oil in a large, ovenproof frying pan over medium-high heat. Season salmon fillets to taste with salt and pepper. Sear salmon until golden brown on both sides, 30 to 40 seconds per side. Transfer to a plate, and wipe excess oil from frying pan. Spread honey over top of fillets, then evenly spread with reserved ginger crust.
Replace salmon into the frying pan, and place into the oven. Bake until the centre of the salmon turns opaque, 6 to 7 minutes.
Meanwhile, melt 2 tablespoons butter in a small saucepan over medium heat, stir in shallots, and cook until softened and translucent, about 2 minutes. Pour in white wine, then simmer until nearly evaporated. Add the ponzu sauce and double cream, reduce by 1/2. Remove from heat, then whisk in cold butter pieces until dissolved; season to taste with sea salt.
To assemble the dish, season the cucumber salad to taste with salt. Divide the salad evenly onto four plates, spreading it out into a 8cm circle. Drizzle the ponzu butter onto the bare area of the plate, around the cucumber salad. Place a salmon fillet, crust-side up, on top of the salad, and serve.
Ponzu sauce is a citrus-based sauce common in Japanese cuisine. Look for it online or in specialty shops.