Made with a creamy Mornay sauce of Gruyère and Parmesan cheese, this delicious dish makes a great springtime side or vegetarian entrée.

Preparation

Preheat oven to 450°F.

Melt 3 tablespoons of butter in a 2-quart sauce pot over medium heat. Add flour and whisk to combine. Cook for 1 minute, then slowly add milk while whisking constantly to avoid lumps and scraping the edges of the pot to prevent scorching. When all of the milk has been incorporated, add nutmeg and simmer over low heat for 5 minutes.

Remove pot from heat and add the Gruyère and Parmesan. Whisk until all of the cheese has melted and the sauce is smooth. Adjust seasoning to taste with salt and pepper, then reserve warm.

Prepare leek by removing the green top, then halving lengthwise, slicing into ¼-inch pieces and rinsing in a bowl of water. In a separate pan, melt 1 tablespoon butter. Add leek and stew over medium heat until just tender. Turn off heat, then add asparagus and about 8 ounces of sauce and toss to coat. (Add more sauce for a creamier finished dish.)

Spread evenly into a low-sided, oven-safe casserole dish or individual ramekins. Melt 1 tablespoon of butter and mix it with the panko bread crumbs and tarragon. Sprinkle mixture evenly over the top of the gratin and bake or broil until the top is golden brown.