Turn your instant pot onto the 'sauté' function. Add the diced pancetta and cook for 4-5 minutes, stirring regularly until the pancetta has crisped up and rendered some of its fat. Transfer pancetta to a plate and set aside. Add the olive oil to the instant pot and the cubes of beef into a single layer. Let the meat get very brown on all sides. You may have to do this in two batches. Remove the beef from the pan and set aside with the pancetta.

Add the onions, carrots and celery and cook for 4-5 minutes, stirring occasionally until soft. Add the minced garlic and cook 1 minute more. Return the beef and pancetta to the pot. Add the wine, and bring to a simmer. Simmer until the wine is almost completely evaporated and then add the tomatoes and juice, bay leaf, oregano, red pepper flakes, salt, pepper, and milk. Place the lid on the pot and lock into place. Cancel the sauté setting and change to 'meat/stew' high pressure for 1 hour and 15 minutes.

Manually release the pressure on the instant pot (make sure to do this away from walls or shelves that could be damaged by steam). Remove the lid, shred the meat with a fork, and transfer again to the sauté function and simmer for 15 min or so until thickened, stirring once in a while so that the ragu doesn't stick. While you are doing this, cook your pasta and drain. Transfer pasta to bowls, top with ragu, mascarpone, freshly grated parmesan and red pepper flakes (if desired).