In a medium bowl, combine olives, olive oil, parsley, capers, garlic, anchovy, lemon zest, and pepper. Set aside.
In a blender, combine 1/4 cup olive oil, egg, lemon juice, garlic, salt, and red pepper. Blend until combined. Remove center of cover (or entire cover) and, at low speed, slowly pour remaining 3/4 cup olive oil into mixture in a steady stream; continue blending until thick. (Makes about 1 cup.)
Arrange potatoes in a steamer basket set over 1 inch simmering water. Cover and steam until tender, 15 minutes. Cool, then coarsely chop. In a large bowl, combine potatoes, 1/4 cup aioli, scallion, lemon juice, and salt. Reserve remaining aioli.
Sprinkle both sides of tuna with fleur de sel, and drizzle with olive oil. Heat a cast-iron pan or heavy skillet until very hot. Add tuna, and cook until pink in center, about 3 minutes per side for medium-rare, or until desired doneness. Set aside to cool. With a sharp knife, cut tuna into 1/4-inch-thick slices.
Scatter watercress over a large platter. Place potato salad over watercress and arrange pieces of tuna on top. Spoon tapenade over fish and around potatoes.
Recommended ingredients: Ground coriander and fennel may be substituted for whole seeds. Davidson's pasteurized eggs are available online at safeeggs.com. Fleur de sel is available online at chefshop.com or at specialty food markets.