Preheat oven to 375°F. Melt 3 tablespoons butter in large skillet over medium heat. Sauté onions and garlic until golden brown. Transfer to a small dish and combine with cornstarch.
Melt additional 3 tablespoons butter in skillet; sear pork, evenly browning on all sides. Transfer pork to 11×7-inch baking dish.
Deglaze skillet with vinegar. Stir in onion mixture, water, sugar, red pepper, salt, rosemary and cranberries. Cook mixture until the sauce thickens and the cranberries begin to split.
Pour sauce over pork and roast until thermometer inserted into center registers 155°F (about 25 minutes). Baste and turn every 10 minutes. Slice pork and serve with sauce. Garnish with fresh rosemary sprigs.