1. Adjust oven rack to middle position; heat oven to 300 degrees. Thoroughly pat beef dry with paper towels; sprinkle generously with salt and pepper. Place pancetta in 8-quart heavy-bottomed Dutch oven; cook over medium heat, stirring occasionally, until browned and crisp, about 8 minutes. Using slotted spoon, transfer pancetta to paper towel-lined plate and reserve. Pour off all but 2 tablespoons fat; set Dutch oven over medium-high heat and heat fat until beginning to smoke. Add beef to pot and cook until well browned on all sides, about 8 minutes total. Transfer beef to large plate; set aside.
2. Reduce heat to medium; add onions, carrots, celery, and tomato paste to pot
and cook, stirring occasionally, until vegetables begin to soften and brown,
about 6 minutes. Add garlic, sugar, flour, and reserved pancetta; cook, stirring
constantly, until combined and fragrant, about 30 seconds. Add wine and
tomatoes, scraping bottom of pan with wooden spoon to loosen browned bits; add
thyme sprig, rosemary, and parsley. Return roast and any accumulated juices to
pot; increase heat to high and bring liquid to boil, then place large sheet of
foil over pot and cover tightly with lid. Set pot in oven and cook, using tongs
to turn beef every 45 minutes, until dinner fork easily slips in and out of
meat, about 3 hours.
3. Transfer beef to cutting board; tent with foil to keep warm. Allow braising
liquid to settle about 5 minutes, then, using wide shallow spoon, skim fat off
surface. Add minced thyme, bring liquid to boil over high heat, and cook,
whisking vigorously to help vegetables break down, until mixture is thickened
and reduced to about 3 1/2 cups, about 18 minutes. Strain liquid through large
fine-mesh strainer, pressing on solids with spatula to extract as much liquid as
possible; you should have 1 1/2 cups strained sauce (if necessary, return
strained sauce to Dutch oven and reduce to 1 1/2 cups). Discard solids in
strainer. Season sauce to taste with salt and pepper.
4. Remove kitchen twine from meat and discard. Using chef's or carving knife,
cut meat against grain into 1/2-inch-thick slices. Divide meat between warmed
bowls or plates; pour about 1/4 cup sauce over and serve immediately.