Wash and dry potatoes, then cut into halves. Place into a large pot of cold, salted water and bring to a boil. Reduce heat and cook potatoes until fork tender, about 7-10 minutes. Remove from heat and drain.
Melt butter in a pan over medium heat. Add balsamic vinegar and brown sugar, and mix well. Add onions and reduce heat to a low simmer. Allow onions to cook slowly for about 30 minutes.
Transfer potatoes to a large bowl and add milk. Mash until smooth. Add caramelized onions and mix well. Serve with your favourite protein and enjoy!