Coat a small skillet with cooking spray; place over medium-high heat until hot.
Add onion, mushrooms, and garlic to skillet, saute until render, and set aside.
Cook spinach according to package directions, omitting salt.
Drain well, and place on paper rowels; squeeze until barely moist.
Com- bine spinach, breadcrumbs, Parmesan, egg, rhyme, and 1/2 teaspoon pepper in a small bowl; stir until well blended.
Set mixture aside.
Lay turkey breast flat, skin side up, on waxed paper.
Carefully slice away tendons, skin, and excess far, keeping meat intact.
Turn turkey breast over carefully to keep center of rurkey breast attached.
Beginning at center of turkey breast, slice horizontally through thickest parr of each side of breast almost to outer edge, flip the cur pieces of breast fillers over and our to enlarge breast surface area and to make a more even thickness.
To fill in shallow area between breast halves, slice meat horizontally from thickest parrs of breast, and lay slices in shallow area.
Pound breast to 1/2-inch thickness.
Spread reserved spinach mixture over turkey breast, leaving a 1/2-inch border at sides.
Roll up turkey, jellyroll fashion, starting at long side.
Sew up seams, using a needle and thread, or secure with wooden picks, if necessary.
Combine sage, salt, white pepper, and paprika in a small bowl; stir well, and rub over outer surface of turkey.
Insert meat thermometer into turkey breast, making sure end touches meat.
Place in a browning bog prepared according to package directions.
Place in a shallow roasting pan.
Bake at 350° for 1 hour or until meat thermometer registers 170°.
Transfer to a serving platter, let stand 15 minutes.
Remove thread or wooden picks.
Slice with an electric knife.
Garnish with thyme sprigs before serving.