Saute mushrooms in a skillet with 1 tablespoon of olive oil until soft. Keep warm in pan.
Saute spinach in pan with 1 tablespoon of olive oil until soft. Keep warm in pan.
Peel potatoes. Cut off sides of potatoes. Add potatoes to a pot of boiling water with saffron, for color and taste. Boil until tender. Drain.
Season the fish with white pepper and salt on both sides. Heat a non-stick skillet over high heat. Brush fish on both sides with olive oil. Add fish to skillet and sear on both sides, 3 minutes each side.
Using only the white part of the leek, halve lengthwise. Take each half and julienne thinly. Wash and dry. Deep fry in hot oil for 1 to 2 minutes until crisp. Drain on paper towels.
Put spinach in center of plate. Put mushrooms on spinach. Put whitefish on top of mushrooms. Cover fish with Mousseline Mustard Sauce. Put 2 potatoes on side of plate. Put a small heap of leeks on top of fish.
Mousseline Mustard Sauce:
Combine in a saucepan, shallots, white wine, vinegar, salt and pepper. Reduce to 10 percent of its original volume. When reduced, add heavy cream. Slowly whisk in soft butter, spoon by spoon. Fold in mustard. Adjust seasoning, to taste.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.