Mushroom Leek Gratin

01Method

1. 1

   Heat the oven to 400°F (200°C).

2. 2

   In a large skillet over medium heat, melt the butter. Stir in the leeks and celery root and let them soften for 10 minutes. Add the diced mushrooms, half the tarragon, and half the chives; cook until the mushrooms have released their juices, about 6 minutes.

3. 3

   While the vegetables cook, toss the panko and Gruyere together in a bowl until combined.

4. 4

   Dust the flour over the mushroom mixture and stir until fully incorporated, about 1 minute. Gradually stream in the heavy cream, stirring constantly, until the sauce thickens and becomes smooth, about 3 minutes. Fold in half of the panko-cheese mixture and stir until the cheese starts to melt, about 2 minutes.

5. 5

   Take the pan off the heat and spoon the mixture into a greased baking dish. Scatter the remaining tarragon, chives, and panko-cheese mixture on top, then drizzle with a little water to moisten the topping.

6. 6

   Bake until the cheese is melted and bubbling, about 10 minutes, then serve right away.