Coat a large saute pan with olive oil, add the pancetta and bring the pan to a medium heat. Cook the pancetta until it gets brown and crispy, about 5 to 7 minutes. Add the onions and crushed red pepper and toss to incorporate with the pancetta. Season with salt, to taste. Cook the onions until they are soft and very aromatic, about 7 to 8 minutes.
While the onions are cooking, in a large bowl, combine the bread and the milk. Toss to combine and let sit until the bread has absorbed the milk and is very soft. Use your hands to get in there and really squish everything together. Reserve.
Add the garlic to the pan with the pancetta and onion and saute for 2 to 3 minutes. Add the mushrooms, season with salt, to taste, and saute until the mushrooms are soft and have let off their moisture, about 4 to 5 minutes. Remove the mixture from the heat and stir in the spinach.
Add the onion/mushroom mixture to the reserved bread and stir to combine. Add the pine nuts, provolone and Parmigiano-Reggiano, and stir to combine. Taste to make sure that the mixture is delicious and season with salt, to taste, if needed. Set aside.
Lay the beef slices between 2 pieces of plastic wrap and gently pound with a meat mallet to flatten and even out the slices. Put about 1/4 cup of filling on 1 end of each of the pounded beef slices and roll up. Secure the rolls with toothpicks. Repeat this process with the remaining beef and filling.
Coat a large, wide pot with olive oil and put over medium-high heat. Season the beef rolls with salt, to taste, and brown them on all sides. When the beef rolls are brown on all sides, remove them from the pan and reserve. Make the sauce in the same pot.
Remove the oil from the pot that the beef was just browned in. Add a light coating of fresh olive oil and add the onions and crushed red pepper. Season with salt, to taste, and put the pot over medium heat. Sweat the onions until they are translucent and very aromatic, about 7 to 8 minutes. Add the garlic and saute for another 2 to 3 minutes. Stir in the tomato paste and red wine, and cook for 6 to 8 minutes. Add the tomatoes, and 2 cups of water and season with salt, to taste. Return the beef rolls to the pan and snuggle them into the sauce. Bring the sauce to a boil, then reduce the heat and simmer until the beef is very tender and flavorful, about 2 to 2 1/2 hours. Remove the toothpicks before serving. To serve, arrange 2 or 3 braciole on each serving plate. Halve 1 or 2 rolls to expose the stuffing. Spoon on some of the sauce and garnish with Parmigiano-Reggiano and chopped parsley.