Preheat oven to 200 degrees F. Gently boil potatoes until tender. While this occurs, heat oil in a skillet set over medium-high heat. Season salmon with salt and pepper. Sear 3-4 minutes on each side, and then remove and place on a heatproof plate. Keep warm in the oven. Remove excess oil from skillet and then add lemon juice and stock and reduce by half. Add whipping cream and simmer until sauce lightly thickens. Stir in chives and season with salt and pepper. When potatoes are tender drain well and mash. Whip in buttermilk, butter, salt and pepper until well combined. To serve, divide and mound potatoes on 4 plates. Top with salmon and drizzle with sauce.