1. Make roast: Using a small, sharp knife, make slits all over beef roast and insert a piece of garlic deeply into each slit, so meat closes over garlic. In a small bowl, combine chopped thyme, mustard, onion powder, salt, pepper and olive oil. Rub mixture all over meat, place roast on a plate and refrigerate uncovered for 8 hours or overnight.
2. Transfer roast from refrigerator to countertop 1 hour before cooking. Preheat oven to 500ºF. Scatter onion wedges and thyme sprigs over bottom of a large roasting pan and place a rack on top. Place roast in rack. Roast meat for 15 minutes, then reduce oven temperature to 325ºF and roast until a meat thermometer inserted in thickest part of roast registers 125ºF for medium rare, about 1 hour, or 135ºF for medium, about 1 hour 15 minutes. Check once or twice during roasting time; if juices start to scorch on bottom of pan, add a little broth or water. Remove meat to a platter, cover loosely with foil and allow to rest. (Meat will continue to cook while it rests; internal temperature will rise to 130ºF for medium rare or 140ºF for medium.)
3. Make gravy: Remove rack from roasting pan. Add 3 Tbsp. oil to pan. Place pan over 2 burners and turn heat to medium. Stir in flour and cook, whisking, until brown, 2 to 3 minutes. Pour in wine and broth and bring to a boil, scraping up browned bits from bottom of pan. Reduce heat to low and simmer for 10 minutes, whisking often. Strain through a fine-mesh sieve into a serving dish and season with salt and pepper.
4. Remove strings from roast, slice meat and serve with gravy.