How to Make It
Cut the tuna against the grain into 1/2-inch-thick strips. Cut each strip into 1-inch-long pieces; you should have 24 pieces of tuna.
In a medium bowl, combine the soy sauce and olive paste. Add the tuna and gently stir to coat. Cover and let marinate in the refrigerator for at least 1 hour or up to 3 hour.
Heat the olive oil in a large skillet. Pat the tuna dry with paper towels and add half the pieces to the skillet. Cook over high heat, turning once, until well browned, about 15 seconds per side. Transfer to paper towels to drain. Sear the remaining tuna pieces in the skillet. Let cool, then cover and refrigerate for at least 2 hours or up to 1 day.
Chop the olives with 1 tablespoon of the capers to form a chunky paste. Transfer to a small bowl and stir in the mayonnaise.
Cut the potatoes crosswise into 24 rounds, about 1/4 inch thick. Put the rounds in a medium saucepan of cold salted water and bring to a boil over high heat. Lower the heat to moderate and cook the potatoes until tender, 7 to 10 minutes. Using a slotted spoon, transfer the potatoes to paper towels to drain.
Bring the cooking water back to a boil and add the haricots verts. Boil over moderately high heat until tender, about 1 minute. Drain the beans, rinse with cold water and transfer to paper towels to drain. Cut 1 inch from both tips of each bean and reserve for the garnish.
Spoon 1/2 teaspoon of the olive mayonnaise on each potato slice. Set a piece of tuna on top and garnish each with 1/2 teaspoon of the diced tomatoes, a few capers and 2 haricot vert tips.