Cook the bacon in a large skillet over medium heat until crispy. Using a slotted spoon, transfer the bacon to a plate covered with a paper towel to drain. Once it has cooled enough to touch, crumble it into small pieces and set aside.
Drain off all but about 1 tablespoon of the bacon grease. Return the pan to the stove and cook the onion over medium heat until it is soft and translucent, about 6 to 8 minutes. Add the Brussels sprouts and a pinch of salt and pepper to the pan, and cook the sprouts until the edges are crisped and they are cooked through, about 10 to 12 minutes.
Scrape the Brussels sprouts into a serving dish and add the crumbled bacon and cranberries. Toss gently and serve.