2 cups asparagus tips, cut into 1-inch pieces
4 tablespoons butter
½ cup finely chopped yellow onion
1 cup milk
3 tablespoons flour
4 egg yolks
¾ cup freshly-grated Parmesan cheese
Salt and pepper to taste
5 egg whites
Cream of tartar or lemon juice
Preheat oven to 425 degrees.
Bring a kettle of salted water to a boil, drop in the pieces of asparagus, and cook until tender-crisp. Drain and plunge into ice water. When cool, drain, and puree in a food processor.
Melt butter in a heavy saucepan, and sauté the onion, partially covered, until tender and lightly colored, about 25 minutes. In a second saucepan, bring the milk to a boil. Sprinkle the flour over the onion and butter and stirring constantly, and cook gently for 5 minutes. Shut off the heat and pour in the boiling milk all at once. Beat vigorously with a whisk as the mixture bubbles, and then set the pan over medium heat. Bring sauce to a boil, stirring constantly, and let it boil for 3 minutes.
Remove from heat. Stir in egg yolks, one at a time. Then stir in the asparagus puree, ¼ cup plus 2 tablespoons of the Parmesan, and salt, pepper and nutmeg to taste. Taste and correct seasoning. Butter a 1-quart soufflé dish and sprinkle with remaining Parmesan. Shake out excess cheese and reserve it.
Beat egg whites until foamy. Add a pinch of salt and either a pinch of cream of tartar or a few drops of lemon juice and continue to beat until the whites form stiff peaks. Scoop out about a third of the whites and stir them thoroughly into the asparagus-egg mixture. Scrape the remaining beaten egg whites onto the asparagus mixture and fold in just until incorporated. Do not over mix.
Pour the soufflé mixture into the prepared dish and rap the dish on a work surface to eliminate air bubbles. sprinkle the top with the reserved Parmesan. Set the soufflé in the center of the preheated oven and reduce the heat to 375 degrees.
Bake the soufflé 20 minutes without opening the oven door, then check the soufflé. It should be browning well and should have risen 2 inches above the rim of the dish. Let it bake for another 15 or even 20 minutes if you like a very well-done soufflé. Serve immediately.