Could you recommend some variations to the topping for the fennel gratin, apart from the cheese and breadcrumbs mixture mentioned in the recipe?: Rinse the fennel and remove any scuffs. Cut the fennel lengthwise into strips with the width of about a half-inch. Place them in a pan over medium heat with the milk, water, salt, and butter, and cook uncovered until al dente, 10 to 15 minutes. (You may need to reduce the heat to medium-low once it heats up.) Set aside when cooked.
Set the broiler at medium-high heat.
Put the grated cheeses in a bowl with the breadcrumbs and mix together.
Butter a baking pan (or use a nonstick pan). Remove the cooked fennel from the milk mixture and place in the baking pan (discard the milk mixture). Pour the Mornay sauce over the fennel, sprinkle the top with the cheese and bread mixture, and cook it under the broiler until crisp on top.
This prompt doesn't seem malicious.