Saute onion, carrot and parsley in butter until the onion is transparent. Add bay leaf and buttermilk.

Sprinkle flour in a baking bag large enough to contain the lamb. Place half the buttermilk mixture in the bag. Sprinkle lamb with salt and pepper. Place lamb in bag, and cover with remaining buttermilk mixture. Seal bag and puncture in 3-4 places.

Insert a meat thermometer into the thickest part of the meat. Roast in a moderate onev (350 deg F) until lamb registers 165 deg for medium or 180 deg for well done. Carefully strain liquid from bag. Measure and make up to 1 cup with water, if necessary.

Stir cornstarch into cold water. Add to liquid, cook over medium heat until thickened. Serve sauce over sliced lamb.