Fill a large pot with water and bring to a boil over high heat. Generously salt the water and add the Brussels sprouts. Simmer the sprouts until just tender, about 5 minutes. Do not overcook. Drain the sprouts well.
Heat a large skillet over medium heat and saute the bacon until browned and crisp, stirring occasionally.
Remove the bacon from the pan to a plate lined with a paper towel. Crumble the bacon and set aside.
Leave the bacon fat in the pan and add the red onions and 1 tablespoon of butter. Saute until the onions are translucent, 2 to 3 minutes. Add the cranberries and cook for 1 minute. Stir in the red wine vinegar and brown sugar and cook until the liquid is slightly reduced, about 3 minutes.
Add the sprouts to the skillet and stir to coat with the vinegar mixture. Add the remaining 1 tablespoon butter. Stir in the reserved bacon. Season with salt and freshly ground black pepper, to taste, and transfer to a bowl to serve.