Cook the pancetta in a heavy large oven-safe pot over medium heat until crisp, about 8 minutes. Add the onions and garlic, and saute until the onions are translucent, about 5 minutes. Mix in the beer, tomato sauce, brown sugar, vinegar, molasses, mustard, salt, and pepper. Stir in the beans. Bring to a simmer. Transfer to the oven and bake, uncovered, until the bean mixture bubbles and thickens slightly, stirring occasionally, about 45 minutes.