Preheat oven to 375 degrees F. Wrap onions individually in foil. Place onions in preheated oven and bake about 1-1/2 hours or until onions are very soft. Remove from the oven and let cool. Remove foil, peel onions, and coarsely chop. Set aside or cover and chill up to 24 hours.
Trim the base of the Brussels sprouts as minimally as possible. Remove any damaged leaves. Cook Brussels sprouts, covered, in a large saucepan in lightly salted boiling water for 8 to 10 minutes or until tender. Drain. Plunge into ice water until they are cool. Drain Brussels sprouts and cut in half by cutting through the bottom of the sprouts. Set aside or cover and chill up to 24 hours.
In a very large skillet, cook garlic in hot oil for 30 seconds. Add Brussels sprouts and chopped onion. Cook and stir for 5 to 10 minutes or until heated through. Add Miso Vinaigrette and stir well to combine. Transfer to a serving dish; sprinkle with parsley.
In a small bowl, use a wire whisk to combine red miso, red wine vinegar, and ground black pepper until smooth. Gradually whisk in olive oil until well combined. Stir in finely chopped green onion.