Preheat broiler to high. Line a baking sheet with aluminum foil.
Rinse short ribs and pat dry. Sprinkle with salt. In a large bowl stir together the mustard, sugar, garlic, and vinegar. Add short ribs and toss to coat. Transfer to the baking sheet and plce in broiler. Cook until bnrowned on top, about 4 minutes, then turn and brown on other side.
Set oven temperature to 375 degrees.
Put bacon in a 5 to 7 quart dutch oven and set over medium high heat. Cook, stirring as needed, until brown but not completely crisp. Pour off fat and pour beer into pot. Bring to boil and scrape bootom of pan to deglaze.
Transfer ribs to pot and sprinkle with salt and pepper, then scatter onion wedges over all. Cover pot and place in oven. Cook until meat pulls easily away from bone with fork, about 1 hour and 30 minutes, stir contents of pot after first hour.
Transfer pot to stove top and lift out meat and bones, set aside. Tilt pot to one side and skim off as much fat as possible. There should be very little sauce at this point, if it is still liquid, place pot over high heat and reduce until only a few tablespoons remain. Sprinkle flour over pot and cook, stirring, over high heat for 30 seconds. Pour broth in pot and bring to a boil. Cook, stirring, until sauce thickens, about a minute or two. Return short ribs to pot and stir to coat with sauce. Remove from heat and let sit for 10 minutes before serving.