Make roast: Two hours before cooking, remove meat from refrigerator and set on a countertop to bring to room temperature.
Preheat oven to 475°F. Combine flour, dry mustard and onion powder and pat onto roast. Season generously with salt and pepper. Place roast in large roasting pan, bone side down. Place a piece of onion at each corner of meat. Roast meat for 15 minutes, then reduce oven temperature to 375°F and roast another 15 minutes per pound, approximately 1 hour total, until a meat thermometer registers 135°F for medium rare or 145°F for medium.
Remove roast from oven to platter, tent loosely with foil and let rest for 20 to 30 minutes. Roast's internal temperature will rise about 5°F to 10°F as it rests. When ready to serve, transfer to a cutting board and carve into thick slices.
Make gravy: Remove onions and all but 3 Tbsp. of fat from roasting pan. Place pan on 2 burners over medium heat, sprinkle in flour and cook, whisking constantly, for 2 minutes. Add wine and broth and bring to a boil, stirring and scraping up browned bits from bottom of pan. Reduce heat and simmer until gravy reaches desired consistency. Season gravy with salt and pepper and add any juices that accumulated on platter while meat was resting. Strain gravy and serve with roast.