Ingredients for 6 servings:
½ cup chervil leaves and tender stems
3 tablespoons unsalted butter at room temperature
1 quart water
2 pounds asparagus (30 + spears)
1/3 cup all-purpose flour
1-1/2 cups asparagus cooking water
1 tablespoon lemon juice
Kosher salt, freshly ground white pepper to taste
2 large egg yolks
Chervil butter: Chop the chervil and blend with the softened butter, preferrably the day before making the gratin.
Bring the water to a simmer in a deep skillet. Rinse the asparagus spears. Break off the stem ends and discard. Cut off and reserve the asparagus tips. Cut the remaining stalks into 1” sections and reserve.
Blanch the asparagus tips in the boiling water until just tender. Remove and submerge in ice water until cooled. Drain and reserve. Add the stem pieces and repeat this process. After the asparagus has been removed continue boiling the cooking water until reduced to 1-1/2 cups. Reserve.
Preheat the oven to 275 degrees. Line the bottom of an 8” gratin dish with the asparagus tips filling in with stem pieces as needed.
Melt the herb butter in a deep skillet. Stir in the flour and cook for two minutes stirring steadily. Pour in the hot asparagus broth and whisk until the white sauce is smooth and homogenous. Off the heat, season to taste with salt and pepper. Whisk in the eggs, one at a time.
Spoon the sauce evenly over the asparagus tips in the dish. Bake for 30-40 minutes until set. Keep warm in the turned off oven.
Just before serving preheat the broiler. Place the gratin dish 4” from the flame and lightly brown the top. Serve immediately.