- Position rack in lower third of oven and preheat to 400 degrees. Generously butter 4- or 6-cup** soufflé dish. Sprinkle with 2 tablespoons of Parmesan cheese to coat, and shake out excess. (**If using 4-cup dish, tie a collar around rim, see below).
- Bring a pot of salted water to a boil, drop in asparagus, and cook, uncovered, until just tender, 3 to 4 minutes.Drain and allow to cool before puréeing in food processor until smooth.
- Melt butter in a medium saucepan over medium-low heat and sauté shallots about 5 minutes. Stir in flour, cayenne, and nutmeg. Cook, whisking constantly to prevent browning, for 1 minute. Gradually whisk in milk, bring to a low boil, and whisk for about 3 more minutes until smooth and thick. Remove from heat.
- Mix yolks in a small bowl with ½ teaspoon salt and black pepper to taste. Whisk yolks into sauce until blended. Mix in asparagus purée, Gruyère, and remaining Parmesan. (Note: up to this point can be prepared a few hours in in advance).
- Using electric mixer, beat whites, pinch of salt, and cream of tartar until stiff but not dry. Mix ¼ of whites into lukewarm soufflé base to lighten. Gently fold in remaining whites. Pour mixture into prepared soufflé dish.
- Place soufflé in center of oven and immediately reduce heat to 375 degrees. Bake without opening oven door for 20 minutes. Check for browning and rising, and then bake another 15 to 20 minutes until golden and gently set in center. Place soufflé dish on a platter and serve immediately.
*Asparagus should have tough stem ends cut off and discarded and thick stalks peeled.
**If using four-cup soufflé dish, make a collar: cut a length of parchment paper or aluminum foil long enough to wrap around the dish with a 2-inch overlap. Fold in half lengthwise and butter one side. Wrap the collar around the dish, buttered side in. Secure in place with paper clips.