Line an oven safe dish with foil (I use my cast iron skillet). Place half of the onion slices in the pan, top with a couple of tablespoons of the raspberry preserves. Salt and pepper the pork loin, rub the sage, thyme and garlic into the pork loin. Place fat side up on top of the onions/preserves. Top with the remaining onion slices and a couple more tablespoons of the raspberry preserves. Fold the foil over and bake at 375 for about 35 minutes. Open foil, spoon drippings over the top of the pork loin and cook until top is browned (about another 15 minutes). Remove from oven and pour the pan juices into a small saucepan. Mix some cornstarch with cold water to dissolve. Pour into the pan juices a little at a time to thicken sauce. Drizzle sauce over slices of the pork loin.