Roast Pork with Sweet Onion Rhubarb Sauce

Name: Roast Pork with Sweet Onion-Rhubarb Sauce
Ingredients: 4 teaspoons extra-virgin olive oil, divided; 1 ½ teaspoons ground coriander; 1 teaspoon kosher salt, divided; ¼ teaspoon freshly ground pepper; 1-1 1/4 pounds pork tenderloin, trimmed; 1 large sweet onion, sliced; 2-4 tablespoons water; 2 cups diced rhubarb; ¼ cup red-wine vinegar; ¼ cup brown sugar; ¼ cup minced fresh chives