Heat the olive oil in a large skillet over medium-high heat. Add onions, salt and pepper to taste; cook, stirring occasionally, until lightly browned, about 5 minutes. Add the mushrooms and cook, stirring occasionally, until browned, about 2 minutes. Stir in the barley and thyme; cook, stirring, until the barley is just golden, about 2 minutes.
Transfer to a 6-quart slow cooker; add carrots, broth, and water. Cover and cook on HIGH until the liquid is absorbed and carrots and barley are tender, about 3 hours. Discard thyme; stir in Parmesan and vinegar. Thin risotto with warm water for desired consistency, as needed. Garnish with parsley; season to taste with salt and pepper. Makes 4 servings.
Approximate Nutrient Analysis per serving (not including salt to taste):
440 calories, 10 g fat, 2.5 g saturated fat, 5 mg cholesterol, 300 mg sodium, 76 g carbohydrate, 17 g fiber, 10 g sugar, 15 g protein