Combine the oil, lemon juice, tarragon, shallots and salt. Toss with the crab. Place in a covered container and refrigerate for several hours to let flavors blend.
To serve, place an asparagus spear-half on each leaf of endive. Top with approximately 2-3 teaspoons Dijon Sauce and sprinkle with crab.
Dijon Sauce: 2 egg yolks, room temperature 1 tablespoon rice wine vinegar 2 teaspoons fresh lemon juice 5 teaspoons Dijon mustard 1/2 teaspoon sugar 1/4 teaspoon slat 1 Dash of white pepper 1/2 cup vegetable oil 1/2 cup Creme Fraiche
Place the egg yolks, vinegar, lemon juice, mustard, sugar, salt and pepper in a blender. Blend for a few seconds. With the motor still running, add the oil in a slow steady stream until well incorporated.
Remove the sauce to a bowl and whisk in the Creme Fraiche. Cover and refrigerate. This can be made several days in advance and kept in the refrigerator. Leftover sauce is wonderful with chilled artichokes.