How to Make It
In a small bowl, whisk together the egg yolk and mustard. Start adding the cooking oil a few drops at a time, whisking constantly. When the sauce begins to emulsify, add the rest of the cooking oil and 2 tablespoons of the olive oil in a thin stream, still whisking. Whisk in the lime juice, 1 3/4 teaspoons of the lemon juice, the fish sauce, shallot, capers, Tabasco sauce, 1/2 teaspoon of the salt and 1/4 teaspoon pepper.
Cut the salmon fillet into small dice. Continue cutting the fish until chopped very fine. Put the salmon in a bowl and stir in the sauce and the chopped chives, parsley and tarragon, if using.
Put a slice of smoked salmon on a large tray or a cookie sheet. Put one sixth of the salmon tartare in the middle of the slice and shape the tartare into a patty. Fold the smoked salmon over the salmon tartare. Repeat for the remaining servings. Cover and refrigerate until ready to serve.
In a large bowl, whisk the remaining 2 tablespoons olive oil, 2 teaspoons lemon juice and 1/4 teaspoon salt and a pinch of pepper. Add the pea shoots and toss. Divide the pea shoots among six serving plates.
With a wide metal spatula, invert one salmon-tartare package onto each pile of greens. Top with a small scoop of caviar and serve at once with the toast.
Asian fish sauce and baby pea shoots are available at Asian markets and specialty-food stores.
For the best results, use the belly portion from the salmon. It is the richest part of the fish and is ideal for a tartare.