Peel and chop the onion. Finely dice the gherkins. Wash dill, shake dry, chop finely. Mix mayonnaise, mustard, 2 tablespoons lemon juice, onion, gherkins and dill. Season to taste with salt and pepper.
Sort the spinach, wash and spin dry. Rinse sprouts, drain. Wash the asparagus, cut the woody ends off generously, cut the shoots in half. Rinse salmon, dab dry. Heat 2 tablespoons of oil in two pans. Fry the asparagus in one pan and the salmon in the other for about 6 minutes. Sprinkle with the rest of the lemon juice, season with salt and pepper.
Cut the buns in half. Cover lower halves with spinach, salmon, asparagus and sprouts. Drizzle with sauce, place upper halves on top.