Thinly slice the steak across the grain into planks about ¼-inch thick. Stack a few on top of each other, then cut thin strips that are ¼-inch thick. Cut the strips into a ¼-inch dice. Place the steak cubes in a mixing bowl.
To the steak, add cooled melted butter, white onion, cornichons, chopped capers, Worcestershire, Dijon mustard, and garlic clove. Season with a generous pinch of Kosher salt and a few grinds of freshly ground black pepper, then stir very well. Taste.
Spoon over toasted grilled bread or serve on top of baby arugula. Garnish with thinly sliced chives and a pinch of flaky sea salt. Serve with additional coarse grain mustard and cornichons on the side.