Sauté pancetta in olive oil until crispy.

Add onions and red pepper flakes; season with salt and cook until softened.

Soak bread in milk until soft.

Mix soaked bread with garlic, sautéed pancetta and onion mixture, mushrooms, spinach, pine nuts, provolone, and Parmigiano-Reggiano.

Season the filling to taste.

Pound beef slices to flatten.

Place filling on each slice and roll up, securing with toothpicks.

Brown beef rolls in olive oil over medium-high heat.

Remove beef and set aside.

Sauté onions and red pepper flakes in olive oil until translucent.

Add garlic and sauté.

Stir in tomato paste and red wine and cook for 6-8 minutes.

Add tomatoes and water and season with salt.

Return beef rolls to the sauce.

Bring to a boil, reduce heat, and simmer for 2 to 2.5 hours, or until beef is tender.

Remove toothpicks before serving.

Serve braciole with sauce, grated Parmigiano-Reggiano, and chopped parsley.