How to Make It
Preheat oven to 375°. Stir together egg yolk and cream in a small bowl. Place puff pastry on a parchment-lined baking sheet, and cut 1/2-inch-wide strips off each side. Lightly brush each piece with egg wash. Place cut pastry strips on top of puff pastry to form raised edges, trimming as needed. Prick center of pastry with a fork. Chill 30 minutes.
Heat 2 tablespoons olive oil in a large skillet over medium heat; sauté onion 10 minutes, stirring frequently. Reduce heat to low, add sugar and vinegar, and cook, stirring frequently, 10 minutes or until onion is a dark caramel color. Set aside.
Purée arugula, garlic, and anchovies, if desired, in a food processor. Slowly add remaining 3 1/2 tablespoons olive oil; process until mixture is thick and creamy. Stir in breadcrumbs and next 3 ingredients.
Spread arugula mixture over puff pastry, and top evenly with onion. Bake 30 minutes or until tart edges are golden brown and bottom is firm. Let cool to room temperature. Top with olives, and sprinkle with additional arugula, if desired. Cut tart into squares.