Preheat oven to 450 degrees F.

Mix 1 teaspoon oil, coriander, 1/2 teaspoon salt and pepper in a small bowl. Rub the mixture into pork. Heat 1 teaspoon oil in a large ovenproof skillet over medium-high heat. Add the pork and cook, turning occasionally, until brown on all sides, 5 to 7 minutes. Transfer the pan to the oven and roast the pork until an instant-read thermometer registers 145 degrees F, 15 to 17 minutes. Let rest 5 minutes before slicing.

Meanwhile, heat the remaining 2 teaspoons oil in a large nonstick skillet over medium heat. Add onion and the remaining 1/2 teaspoon salt; cook, stirring occasionally, until browned, 7 to 8 minutes. Add 2 tablespoons water; continue cooking, stirring often, until the onion is soft, 5 to 7 minutes more, adding water a tablespoon at a time if necessary to prevent burning. Stir in rhubarb, vinegar and brown sugar and cook, stirring often, until the rhubarb has broken down, about 5 minutes. Spoon the sauce over the sliced pork and sprinkle with chives.