While the bacon is cooking, heat a large high-sided pan with the bacon fat or coconut oil. Dice the onion and add it to the pan over low-medium heat. Saute until softened, about 5 minutes. Add the garlic for a minute, until fragrant, then add about half the collard greens.
Let the collard greens wilt before adding the remaining collard greens to the pan.
Remove the bacon to a paper towel lined plate when it is done. Let cool a few minutes and then chop into small pieces. Combine the bacon with the wilted greens mixture, then stir in the apple cider vinegar.
Slow Cooker Method:
Add the wilted collard greens mixture to a large slow cooker, along with 2 cups of stock. Cover them and cook on low for 5 – 6 hours.
Turn the heat down to low and add 4 cups of stock. Stir to scrape up any browned bits on the bottom of the pan, then cover and cook until tender, about 1 1/2 – 2 hours.