3. For the mousseline sauce: Whisk mustard, vinegar, and egg yolk together in a medium bowl. Pour in oil in a steady stream, whisking constantly. After a thick mayonnaise forms, slowly add reserved asparagus water, 1 tbsp. at a time, to thin it out. Finished sauce should be slightly thicker than heavy cream (you will need 6-8 tbsp. of the water in all). Fold whipped cream gently into mayonnaise until mixture is smooth. Season with salt and pepper to taste and set aside.