For the leek sauce: Heat a saucepan over medium-low heat. Add the butter, leeks and garlic and cook until soft, about 5 minutes. Turn up the heat to medium and add the vegetable stock, parsley, sea salt and pepper to taste. Let simmer for another 5 minutes. Pour into a food processor and add the dill. Puree and set aside.
For the salmon: Heat the olive oil and butter in a pan over medium heat. Put the salmon fillet into the pan skin-side down and cook for about 8 minutes. Flip and cook until it's sufficiently cooked to your liking, or another 3 minutes.
Place a portion of raw baby spinach on a plate and slide the salmon fillet over the top and drizzle with lots of leek sauce. Squirt with fresh lemon juice and enjoy.