1. Preheat the grill to medium-high heat.
2. Peel and slice the onions 1/2" thick. Rub the onion slices (trying to keep them intact) with the olive oil and sprinkle with salt and pepper.
3. Grill the onions until they are lightly charred and cooked through, about 3 to 4 minutes per side.
4. When the onions are cool enough to handle, chop them roughly and put then in a bowl. Add the balsamic vinegar. (This can be stored in the refrigerator for up to 1 week.)
For the roast pork loin:
1. Preheat the oven to 350°F.
2. Place the pork loin in a roasting pan and sprinkle with the herbs, salt and pepper.
3. Pour the wine into the bottom of the pan and baste occasionally. Roast the pork for about 1 1/2 hours until a meat thermometer reads 160°F.
4. Let the meat rest about 15 minutes before slicing.
For the rhubarb sauce:
1. While the roast is cooking, place all the ingredients for the rhubarb sauce in a pot. Cook, uncovered, over medium heat until the rhubarb is completely soft and falling apart. Keep warm over low heat.
To assemble, place about 1/4 cup of onion relish in the center of a warmed plate. Place several slices of pork on the onions and top with rhubarb sauce.