In a large pot, over high heat, bring the water to a boil. Generously salt the water and add the Brussels sprouts. Cook the sprouts until medium tenderness has been reached, about 8 to 10 minutes.
In the meantime, in a large saucepan over medium heat fry the bacon until crisp, stirring occasionally. Remove 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 3 tablespoons of the butter. Saute until the onions are translucent, about 2 to 3 minutes. Stir in the red wine vinegar and brown sugar and cook until the liquid is slightly reduced, about 5 minutes.
Drain the Brussels sprouts from water and add to pan with the onions. Stir to coat and add the remaining 1 tablespoon of butter. Stir in the reserved bacon and cranberries. Season with salt and freshly cracked black pepper, to taste, and transfer to a serving bowl to serve.