If using bacon, throw in pan and cook over medium-high heat until brown, but still somewhat tender. Pour off all but 1 Tbsp of grease and reserve for other uses. If not using bacon, heat olive oil in a pan over medium-high heat.
Add onions and cook, stirring occasionally, until soft and translucent. Add mushrooms, and wait for them to become very tender. If you've used the bacon, use this time to deglaze the pan with the red wine. If not, leave it out.
Add kale and balsamic and watch like a hawk. You want to sautee the kale, stirring frequently. You want it tender, but not so tender that it loses its bright green color. It should wilt, but not droop — if it droops, you have probably overcooked it.
Once the kale is done, salt and pepper to taste. Serve immediately.