Onion Tomato Relish

Name: Onion-Tomato Relish
Ingredients: 2 tablespoons canola oil; 3 medium yellow onions (about 1 ½ pounds total), finely chopped (about 2 ½ cups); ⅓ cup canned diced plum tomatoes with juice; 1 teaspoon sugar; 3 tablespoons plus 1 teaspoon cider vinegar; 2 teaspoons coarse salt; Pinch of cayenne pepper