Adjust oven rack to middle position and heat oven to 450 degrees. Combine panko and oil in 8-inch nonstick skillet. Toast panko over medium-high heat, stirring often, until golden, about 3 minutes.

Meanwhile, microwave cauliflower and water together in covered bowl until tender, about 3 minutes. Drain cauliflower.

Wipe bowl dry with paper towels. Microwave Boursin, cream, salt, and pepper in cleaned bowl until cheese is melted, about 1 minute. Whisk Boursin mixture until smooth, then add drained cauliflower and toss to coat.

Transfer cauliflower mixture to 8½ by 5½-inch baking dish and sprinkle with toasted panko. Bake until hot and lightly bubbling around edges, about 7 minutes. Transfer gratin tin to wire rack and let cool for 5 to 10 minutes before serving.