In a small saucepan, combine balsamic vinegar, salt, sugar and some water and let it simmer on low heat until the sugar dissolves.
Slice the onions about ½ inch thick.
Add the sliced onions to a glass jar and add the Italian seasoning, red pepper flakes and black peppercorns.
Pour the vinegar mixture on the top such that the onions are able to submerge.
Let sit overnight or in the refrigerator for at least one hour before you use them.
Notes
Remember to let the onions marinate in the vinegar mixture for at least a day before you use them. This will help the onions soak up some of that acidity, and will be crunchy, yet soft.
To get the perfect, thin slices, use a mandoline slicer.
Remember not to overcook the balsamic vinegar mixture. As soon as it starts to simmer, turn the heat down and pour over the onions.
Your leftover balsamic mixture can actually turn into a beautiful salad dressing. Mix it together with some olive oil, minced garlic and seasonings of your choice and you’ve got yourself a total flavor bomb!