Rinse turkey; pat dry. Butterfly breast. Open meat and lay flat between 2 pieces of clear plastic wrap. With a meat mallet pound turkey to Ω-inch thickness (about a 12x10-inch rectangle).
For stuffing, spray a large skillet with nonstick spray coating. Add mushrooms, carrot, celery, green onions, thyme, and pepper. Cook over medium heat till tender. Stir in lemon juice and bouillon granules. Add bread cubes; toss lightly to mix. Stir in enough of the water to moisten. Spoon stuffing over pounded turkey.
Roll up turkey and stuffing starting from a long side. Tuck ends under; tie turkey roll with string. Place on a rack in a shallow baking pan.
Bake in a 350∞ oven, covered loosely with foil, for 1 hour. Remove foil. Bake 15 to 30 minutes more or till a meat thermometer registers 170∞. Remove string. Let stand 10 minutes before slicing. Cut into Ω-inch slices to serve.
