To butterfly turkey breast, begin at one end and cut lengthwise from one side of the breast to within 1 inch of the opposite side. Open meat and lay flat between 2 pieces of clear plastic wrap. With a meat mallet, pound turkey to 1/2-inch thickness (about a 12x10-inch rectangle).
For stuffing, coat a large skillet with nonstick cooking spray. Add mushrooms, carrot, celery, green onions, thyme, and pepper. Cook over medium heat until 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 degree F. oven, covered loosely with foil, for 1 hour. Remove foil. Bake 15 to 30 minutes more or until a meat thermometer registers 170 degrees F. Remove string. Let stand 10 minutes before slicing. Cut into 1/2-inch slices to serve. Makes 8 servings.