Cut each darn of salmon in half lengthways to have 8 pieces.
In a large sauté pan, over medium heat, drizzle the oil. Add the salmon pieces and cook for 2-3 minutes without moving the fish on the pan. Once well coloured, flip the fish on the other side and continue to cook for a further 3-4 minutes without touching the fish. season with salt and pepper.
Then, add the shallot, leek and kale. Cook for a minute or so and add some white wine. Add a lid on the pan and cook for a further 2 minutes until the vegetables and salmon are fully cooked.
Remove the lid and add the cold butter in the pan. Swirl to create a sauce not a pool of melted butter! add a squeeze of lemon if desired and check the seasoning.
Remove from the heat and serve immediately two pieces of fish on a bed of mash potatoes on each serving dish with spoons of kale mixture and the butter sauce.