Prepare onions by cutting an X at the end of the root base. Place all onions in a large bowl and cover with hot tap water. Allow to sit in water bath for approximately 1 hour. (SAVE WATER FOR ONION BROTH)

Next, remove pearl onions from water so you can peel off the outer paper thin skins, leaving the rest of the onion. Transfer to large pot. Add anisette liquor and Port, onion broth, mint springs, and brown sugar; bring to boil, stirring until sugar dissolves. Reduce heat to medium. Simmer until onions are tender when pierced with sharp knife, about 30 minutes. Using slotted spoon, transfer onions to bowl. Boil liquid in pot until syrupy and reduced to 3 tablespoons, about 15 minutes. Discard mint leaves. Stir balsamic vinegar into Anisette and Port reduction. Pour reduction over onions. Serve warm or at room temperature. Allow to marinade before serving, can be made a couple of days ahead. Remove from refrigerator and allow to be served at room temperature before serving.