Preheat oven to 375F.
In a large saute pan over medium heat, heat 2 tbsp butter or oil until melted and beginning to froth. Add mushrooms and cook, stirring frequently, for 2 or 3 minutes. Add garlic, sprinkle with salt and pepper, and continue to cook until most of the liquid has evaporated, 4 or 5 minutes more.
Add 2 tbsp chopped sage and cook 1 minute more. Remove from heat.
Remove skin from turkey breast and set aside (do not discard). Lay turkey breast on a work surface and cover with plastic wrap. Pound with a kitchen mallet to an even 1/2 to 1-inch thickness.
Spread mushroom mixture evenly over turkey breast, leaving a 1 inch border and roll up from the short end tightly into a log. Wrap reserved skin over log and tie at several intervals with kitchen twine.
Place on a broiling pan and rub all over with remaining butter or oil. Sprinkle with remaining sage and season with salt and pepper.
Place pop-up timer, if using, into thickest part of the turkey roll (pierce the skin with a sharp knife if need be).
Roast until timer pops up or until internal temperature reaches 180F, approximately 50 to 60 minutes.