Flatten chicken to 1/4-in. thickness. Spread with mustard; sprinkle with onion. Place asparagus spears down the center of chicken; fold over and secure with toothpicks if necessary. Place seam side down in an ungreased 8-in. square microwave-safe dish. Sprinkle with cracker crumbs. Microwave, uncovered, on high for 6-8 minutes or until chicken juices run clear. Keep warm. For sauce, in a small microwave-safe bowl, melt butter. Gradually whisk in egg yolks, lemon juice, water and salt. Microwave, uncovered, at 30% power for 30 seconds or until mixture reaches 160 degrees F and is thickened, stirr