Portobello Mushroom Gratin

I designed this portobello gratin as sort of a mix between my zucchini gratin and my bell pepper gratin with fennel and mushrooms. It's a robust blend of earthy flavors topped with the classic gratin cheese, Gruyere.

Ingredients

• 4 tablespoons butter
• 2 leeks, diced
• 1 celery root, chopped
• 3 large portobello mushroom caps, diced
• 1 tablespoon chopped fresh tarragon, divided
• 1 tablespoon chopped fresh chives, divided
• 1 cup panko bread crumbs
• 1 cup grated Gruyere cheese
• 1 ½ tablespoons all-purpose flour
• 0.75 cup heavy cream
• 2 tablespoons water, or as needed

Steps

1. Preheat the oven to 400 degrees F (200 degrees C).
2. 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.
3. Meanwhile, combine bread crumbs and Gruyere cheese in a mixing bowl.
4. 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.
5. Bake in the preheated oven until the cheese is bubbly, about 10 minutes.
6. Use a spatula to scoop portions and serve immediately.