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.