Season the beef chunks generously with salt and pepper. Heat olive oil and butter in a large Dutch oven over medium-high heat.

Sear the beef on all sides until deeply browned, about 8 minutes total. Remove and set aside.

Add the pancetta to the pot and cook until the fat renders, about 3 minutes. Add the carrots and celery, cooking for 5 minutes until softened.

Add all the sliced onions to the pot with a generous pinch of salt. Stir well to combine with the vegetables.

Return the beef to the pot, nestling it among the onions. Add the bay leaf and thyme sprigs. Pour in the white wine.

Cover and reduce heat to the lowest setting. Cook for 3.5 to 4 hours, stirring occasionally, until the onions have completely melted into a golden sauce and the beef is fork-tender with an internal temperature of at least 75°C.

Remove the beef and shred it using two forks. Discard the bay leaf and thyme stems. Return the shredded beef to the onion sauce and stir to combine.

Bring a large pot of generously salted water to boil. Cook the rigatoni until al dente according to package directions, about 12 minutes.

Reserve 250ml pasta cooking water before draining. Add the drained pasta directly to the sauce.

Toss everything together over low heat, adding pasta water as needed to achieve a silky coating consistency.

Serve immediately in warm bowls, topped with freshly grated Pecorino Romano and chopped parsley.