In a small bowl, whisk balsamic vinegar, garlic, salt, and pepper until salt dissolves. While whisking constantly add the oil by a thin stream until fully incorporated. Or put all ingredients into a glass jar with a tight-fitting lid and shake vigorously.
In a large salad bowl tear romaine lettuce into bite size pieces. Add the Mesclun Mix, bell pepper, onion, and toss. Add a small amount of dressing and toss gently. Add the toasted walnuts, dried cranberries, and blue cheese crumbles.
Serve immediately with additional dressing on the side.
Notes
Dressing may be made and refrigerated for up to a week ahead. Take dressing from fridge 15 minutes before using.