Remove giblets from hens and chill to simmer for gravy another day. Rinse hens inside and out with cold water, pat dry. Sprinkle cavities lightly with salt.Brush hens, one at a time, with buttermilk, then shake in coating mix. Place, breast side up and not touching, in a jelly roll pan.Bake in a moderate oven (350 F) for 1 hour and 15 minutes, or until tender and golden.While hens bake, cook noodles in a kettle, following label directions. Drain, return to kettle. Add onion salt and butter (or margarine), toss lightly to mix.Spoon noodles onto a large deep serving platter. Arrange Cornish Hens on top. Garnish with sprigs of water cress, if you wish.