Place salmon, skin-side down in a glass baking dish. Rub with sherry. In a mortar grind together salt, sugar and pepper lightly with a pestle. Coat flesh side of salmon with salt mixture. Strew tarragon over salmon, distributing it evenly. Top with another glass dish and weight it with cans. Refrigerate for 48 hours, basting a few times with accumulated liquid.
Cook asparagus in boiling, salted water until just tender, 2 to 3 minutes. Refresh under cold water, then pat dry. Set aside at room temperature.
Prepare sauce: In a small saucepan bring sherry to a boil and reduce it by one-third. Let cool slightly, then pour into workbowl of a food processor with mayonnaise and mustard. Process for 5 seconds. Combine walnut and vegetable oils and, with the motor running, slowly begin to add to sherry mixture drop by drop. After 1 tablespoon of oil has been absorbed, you can begin to pour it slowly in a thin stream. Process to a medium consistency. Season to taste with salt and pepper and set aside at room temperature.
Remove tarragon from salmon and wash off brine under cold water. With a very sharp knife, cut salmon into broad, thin slices. Divide among dinner plates. Pour a little sauce alongside. Top decoratively with asparagus. Grind black