PREHEAT OVEN: to 400 F. Set rack in bottom third of oven. (You can roast vegetables on a top rack if needed.) Drizzle a tablespoon or two of olive oil in the center (a space of about 6” x 12”) of a heavy roasting pan. Set aside.
COOK ONION, GARLIC, AND SAGE: In a small skillet, warm a tablespoon of olive oil over medium flame for a minute and add onion. Cook 5 minutes, stirring. Add garlic and sage and continue cooking until onion is tender. Set aside.
FLATTEN BREAST/LAYER ON PROSCIUTTO AND ONIONS/ROLL UP/TIE: Lay turkey breast out flat and roll or pound briefly with rolling pin and/or meat mallet until breast flattens out a little; it won't be thin or perfectly even. Pat dry all over with paper towels. Salt and pepper the turkey well on both sides and lay skin side down on the cutting board. Layer the prosciutto slices on the breast one piece at a time to cover and then spread the onion-sage-garlic mixture on top evenly. Using both hands, roll breast up (starting at the short end) gently but firmly to form a roll approximately 5-inches thick (depending on weight of breast), placing seam at bottom. Cut four 15″ pieces of cotton kitchen twine. Slip pieces of twine under the turkey roll, spacing the ties out evenly, and tie each together in a knot. Trim excess twine. Carefully place roulade into prepared roasting pan seam side down and drizzle evenly with another tablespoon or two of olive oil.
ROAST/REST/SLICE/SERVE: Place in bottom third of 400F oven and roast about 45-55 minutes OR until instant read thermometer registers at least 160 degrees F. Remove from oven and let rest 10 minutes or until temperature is at least 165. Slice into about eight slices or as you desire. Store leftovers well wrapped in refrigerator for 3 days. Do not freeze.