Preheat the oven to 300°F (150°C) and adjust an oven rack to the lower third position.
Pat the beef dry with paper towels and season with salt and pepper. Heat the oil in a large Dutch oven over medium-high heat until hot but not smoking. Brown the beef on all sides, 7 to 10 minutes, reducing the heat if the beef begins to scorch. Transfer the beef to a large plate.
Pour off almost all the fat in the pot, add the pancetta, and cook over medium heat, stirring often, until browned and crisp, about 8 minutes. Add the onions, carrots, celery, and 1/4 teaspoon salt and cook, stirring often, until softened, 8 to 10 minutes.
Stir in the garlic, 1 teaspoon of the thyme, and the rosemary and cook until fragrant, about 30 seconds. Stir in the flour, tomato paste, and sugar and cook, stirring constantly, for 1 minute. Slowly whisk in the wine, scraping up any browned bits, until smooth. Stir in the tomatoes and bring to a simmer.
Add the beef, along with any juices that accumulated on the plate, to the pot and bring to a simmer. Cover, place the pot in the oven, and cook until the meat is very tender and a fork inserted into it meets very little resistance, 3 to 3 1/2 hours, turning the beef every hour.
Transfer the beef to a cutting board and tent loosely with foil.
Meanwhile, strain the braising liquid into a fat separator or bowl, discarding the solids. Defat the braising liquid by pouring off the fat, skimming the fat with a ladle, or refrigerating the whole shebang overnight so that the solidified fat can be lifted from the surface.
Add the remaining 1 teaspoon thyme to the defatted braising liquid, bring to a simmer over medium-high heat, and cook until thickened, saucy, and reduced to about 1 1/2 cups, 15 to 20 minutes. Season the sauce with salt and pepper to taste.
Remove the kitchen string, slice the beef against the grain into 1/4-inch-thick slices, and transfer to a serving platter with polenta, if desired. Spoon the sauce over the beef, garnish with thyme, if desired, and serve.