Cook bacon and onion in a pan over medium high heat just until onions are soft and translucent.
Add brussels sprouts and continue to cook until they are tender and crisped to your liking.
While brussels sprouts are cooking, add balsamic vinegar to a medium sized pan and bring to a simmer. Allow to simmer until vinegar has reduced to approximately ⅓ cup and is a syrup like consistency then remove from heat.
Add diced apple to the brussels sprouts and cook for an additional 2-5 minutes until apples are slightly softened.