Peel the onions and thinly slice them.

Note: A sharp knife is important. I cut the ends off of mine, cut them in half, peeled the outer layer and placed them cut side down to slice them.

Place the thinly sliced onions in a slow cooker. Add the balsamic vinegar and brown sugar. Cut the butter into smaller pieces and add the butter. Season with kosher salt, pepper, and crushed red pepper flakes.

Turn the slow cooker on low for 8 to 10 hours, stirring a few times throughout.

Use the caramelized onions to make soups, dip, add to grilled cheese or quesadillas, or as a burger topping.

Note: Extra caramelized onions will stay good in the fridge for a few days in a container with lid, or you can freeze them for up to three months.