Remove both ends of onion and cut in half lengthwise. Turn one piece of onion onto flat side and cut into thin slices, working from one end to another. Repeat until all onions are sliced.

Melt butter in a large skillet over medium heat. Add onion slices to skillet and drizzle olive oil over the top. Season with salt and black pepper, to taste, and stir to combine.

Slowly cook the onions down over medium heat for 45 minutes to an hour. (Total cooking time will depend on how many onions were used, the sugar content in the onions, and how old they are). Stir the onions every 5 or 10 minutes while they are cooking to prevent scorching.

Once the onions are done caramelizing, add balsamic vinegar to deglaze the pan. While the liquid is bubbling, gently scrape the bottom of the pan with a wooden spoon to incorporate the fond (the brown bits on the bottom of the pan) into the onions.

Season with additional salt and black pepper, if desired. Use immediately, or cool and store in the refrigerator for up to one week. Enjoy!