Preheat oven to 500°F Butter a large rimmed baking sheet with 2 tablespoons butter. Melt remaining 4 tablespoons butter in small saucepan; set melted butter aside. 

Whisk eggs and mustard in bowl to blend. Mix panko, Parmesan, salt, and pepper in large bowl. Dip chicken into egg mixture, then panko mixture, coating generously. Place on prepared baking sheet. Drizzle reserved melted butter over chicken. Bake chicken until browned and cooked through, turning once, about 10 minutes. 

Garnish chicken with trimmed watercress; serve chicken with Asparagus with Sauce Maltaise.

Sauce:

Whisk egg yolks, ¼ cup water, and lemon juice in medium metal bowl to blend. Place bowl over large saucepan of barely simmering water. Whisk constantly until instant-read thermometer inserted into mix­ture registers 160°F, about 5 minutes. Transfer yolk mixture to blender. With blender running, gradually pour hot melted butter through opening in top of blender and blend until mixture is thick and creamy. Stir in orange peel. Season sauce with salt and pepper. 

Meanwhile, cook asparagus in large saucepan of boiling salted water until crisp-tender, about 4 minutes. Drain; divide among plates. Spoon sauce over.