Portobello Mushroom Gratin

Preheat the oven to 400 degrees F (200 degrees C).

Melt butter in a frying pan over medium heat. Add leeks and celery root and cook for 10 minutes. Add mushroom pieces, 1/2 of the tarragon, and 1/2 of the chives; continue cooking until mushrooms give off liquid, about 6 minutes more.

Meanwhile, combine bread crumbs and Gruyere cheese in a mixing bowl.

Add flour to the frying pan with the mushroom mixture. Add cream in a stream and stir until it becomes an even color, about 3 minutes. Add 1/2 of the bread crumb mixture and stir until cheese starts to melt, about 2 minutes. Turn off heat. Transfer mixture to a baking dish and top with remaining tarragon, chives, and bread crumb mixture. Add a small amount of water to moisten the mixture.

Bake in the preheated oven until the cheese is bubbly, about 10 minutes.

Use a spatula to scoop portions and serve immediately.