1. Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius). Line a baking sheet with aluminum foil and lay salmon on it.
2. Clean and trim leeks, keeping about one inch (two and a half centimeters) of green. Slice.
3. Heat oil in a large frying pan and sauté the leeks until just softened, about five minutes. Remove from heat and allow to cool for five minutes, then add in salt, pepper, mayonnaise, mustard, and juice. Layer evenly on top of the salmon.
4. Bake for 18 minutes, then broil for two minutes until the leeks are brown and crispy.