Chicken Breasts with Spring Vegetables

To serve 4

12 to 16 small new potatoes
8 small carrots with tops
1 bunch scallions
8 large mushrooms
2 lemons
1 pound asparagus
Two 3-1/4-pound whole chicken breasts, skinned, boned and halved
Salt
Freshly ground black pepper
2 tablespoons chopped fresh tarragon leaves or 2 teaspoons crumbled dried tarragon
Savoy cabbage leaves or lettuce leaves
1 tablespoon chopped fresh parsley leaves for garnish

GARLIC SAUCE
1/2 cup butter
4 egg yolks
1 tablespoon tarragon vinegar
4 teaspoons fresh lemon luice
2 garlic cloves
1/2 teaspoon salt
1 tablespoon chopped fresh parsley leaves
Freshly ground white pepper

Bring several quarts of lightly salted water to a boil in a 3- to 4-quart saucepan. Peel a ring around each potato. Cut off all but 2 inches from the green carrot tops, then peel the carrots and trim them to 3 or 4 inches in length. Trim the scallions, leaving 2 inches of green. Put the potatoes in the boiling water and cook them for five minutes. Add the carrots and scallions, and cook for another five minutes. Drain the potatoes, carrots and scallions.

Trim the ends of the mushroom stems; gently rub the mushroom caps with the cut half of a lemon. Snap or cut off the tough end of each asparagus stalk; then peel the stalk from the base upward with a small knife or vegetable parer.

Fill a large vegetable steamer with water to just below the steamer rack. The steamer must be large enough to hold the chicken and vegetables in one layer without crowding them; if not, steam the food in two batches or on a rack in a pot filled with water to a depth of 1 inch. Place the chicken, asparagus and mushrooms in the center of the steamer rack; arrange the carrots, scallions and potatoes around them. Sprinkle the chicken and vegetables with salt, pepper and tarragon. Cover the steamer, bring the water to a boil over moderate heat, and steam the food for 15 minutes or until the juices of the chicken run clear when a breast is pierced with a knife, and the vegetables are tender.

Meanwhile, to prepare the sauce, first melt the butter in a small saucepan. Place the egg yolks, tarragon vinegar; lemon juice, garlic and the 1/2 teaspoon of salt in a blender or food processor; and mix to incorporate all of the ingredients. With the motor on, slowly pour in the melted butter. When the sauce thickens, stir in 1 teaspoon of chopped parsley. Add salt and white pepper to taste.

Arrange cabbage or lettuce leaves on a serving platter. Place the chicken breasts in the center, and group the vegetables around them. Sprinkle the potatoes and mushrooms lightly with chopped parsley if you like, and garnish the platter with lemon wedges. Serve the garlic sauce separately.