Remove the beef from the refrigerator shortly before cooking while you prepare the vegetables. Dry the surface thoroughly with kitchen paper, then season the meat with approximately ¾ teaspoon of the salt and the black pepper.
Place a large, heavy-bottomed Dutch oven over medium heat. Add the diced pancetta and cook for 5–7 minutes, stirring occasionally, until some of the fat has rendered and the pancetta is lightly golden. Transfer it to a plate, leaving the fat in the pan.
Add the olive oil to the pan and increase the heat to medium-high. Brown the beef in batches if necessary, allowing approximately 3–4 minutes per side until deeply browned. Avoid overcrowding the pot. Transfer the beef to a plate and set aside.
Reduce the heat to medium. Add the onion, carrot and celery to the same pot. Cook for approximately 8–10 minutes, stirring frequently, until the vegetables are softened and lightly golden.
Add the chopped garlic and tomato paste. Stir continuously for approximately 1–2 minutes, allowing the tomato paste to darken slightly without burning.
Pour in the red wine and scrape the bottom of the pan with a wooden spoon to release the flavorful browned bits. Simmer for approximately 3–5 minutes, until the wine has reduced by roughly half.
Add the beef stock, diced tomatoes, passata, rosemary, thyme, bay leaves and red pepper flakes, if using. Return the pancetta to the pot and stir to combine.
Place the browned pieces of beef back into the sauce, together with any juices that have collected on the plate. The liquid should come partway up the meat rather than completely cover it. Bring the mixture just to a gentle simmer.
Cover with a tight-fitting lid and reduce the heat to low. Cook for approximately 2½–3 hours, turning the beef occasionally and checking that the sauce remains at a very gentle simmer. Add a small splash of stock or water if necessary.
The beef is ready when a fork slides into it easily and the meat can be pulled apart with very little resistance. If it still feels firm, continue cooking and check again after 20–30 minutes.
Carefully remove the beef from the pot and place it on a board or large plate. Remove and discard the rosemary stems, thyme stems and bay leaves. Allow the sauce to cool slightly before blending.
Using an immersion blender, blend the sauce until smooth and velvety, keeping the blender head completely submerged to avoid splashing. Alternatively, blend in batches using a heat-safe blender, following the manufacturer's instructions.
If the sauce is too thin, simmer it uncovered over medium-low heat for approximately 10–15 minutes, stirring frequently, until it reaches your preferred consistency.
Remove the pot from the heat and stir in the butter until melted. Taste and adjust the seasoning with the remaining salt and additional black pepper if needed.
Using two forks, pull the beef into large, tender pieces rather than finely shredding it. Return the meat to the warm sauce and stir gently to coat.
Serve immediately with creamy polenta, fresh tagliatelle or mashed potatoes. Finish with chopped parsley and a little freshly grated Parmigiano-Reggiano if desired.