Place the picked over crab in a medium size mixing bowl. Add the mayonnaise, mustard, tarragon, lemon zest, and lemon juice. Stir to combine. Season, to taste, with salt and pepper.
Top each slice of cucumber or leaf of Belgian endive with 1 heaping tablespoon of crab salad. Using a 1/2-inch heart-shaped aspic cutter, cut out roasted red pepper hearts. Garnish each mound of crab with a red pepper heart.
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