1Heat oven to 350ºF. Trim fat from pork. Finely chop rosemary and garlic together. Make 8 to 10 deep cuts, about 2 inches apart, in pork, using sharp knife. Insert small amounts of garlic mixture in cuts. Sprinkle salt and pepper over pork.
2Melt butter in shallow roasting pan in oven. Sprinkle onion over butter. Place pork in pan; drizzle with oil. Insert meat themometer so tip is in center of thickest part of pork and does not rest in fat. roast uncovered 50 to 60 minutes or until thermometer reads 155°F. Cover with tent of aluminum foil and let stand 15 to 20 minutes or until thermometer reads 160°F. (Temperature will continue to rise about 5°F, and pork will be eaiser to carve.)