In a large skillet over medium-high heat, warm the olive oil. Add the onions and 1/8 teaspoon of each salt and pepper, and simmer while stirring occasionally for approximately 5 minutes, or until the onions are gently browned. Add the mushrooms and simmer for about two minutes, stirring periodically, until browned. Barley and thyme should be added and cooked for two minutes while stirring constantly.
Add the carrots, broth, 1 1/2 cups water, and 1/4 teaspoon salt to a 6-quart slow cooker. For around 3 hours on high, covered, the liquid will be absorbed and the barley and carrots will be soft.
Add the Parmesan, vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper after discarding the thyme. If necessary, thin the risotto with warm water to achieve the required consistency. Add parsley on top and salt & pepper to taste.