These slow-cooker onions are an ideal condiment for burgers, steaks, a grilled chicken breast or sautéed fish. They also make a pleasing addition to a cheese board or topping for Brie. They're best eaten cold or warm, but not hot.
Combine the onions, fennel, oil, salt, 1 tablespoon of the thyme, and 2 teaspoons of the oregano in a 5- to 6-quart slow cooker, tossing to coat. Cover and cook on LOW until the vegetables are golden brown, about 12 hours. Uncover and stir gently. Increase the heat to HIGH, and cook, uncovered, until the mixture thickens, about 2 hours, stirring every 15 minutes. Stir in the vinegar, and the remaining 1 teaspoon thyme and 1/2 teaspoon oregano.