Place sun-dried tomatoes in a food processor and add the hot water. Let stand for 5 minutes. Pulse til finely minced.
Add mascarpone and egg to the food processor and pulse until smooth.
Add panko to a large bowl and pour the mixture on top. Add oregano, garlic, and crushed red pepper. Mix to combine and let stand 5 minutes.
Add beef to the bowl and season all over with salt and pepper. (At this point, fry a tiny piece, taste, and then adjust seasonings.)
Fry the meatballs:
Form the meatball mixture into big tablespoon-sized meatballs. You should have about 18 meatballs.
Heat one tablespoon of neutral oil in a wide pot. Add the meatballs in an even layer and fry for a few minutes per side until browned all over.
Remove from pot. Set aside.
Prepare the sauce:
To the pot, add diced onion and cook for 7-10 minutes. Add minced garlic and then whole peeled tomatoes and crushed tomatoes. Bring to a boil, reduce heat, add the meatballs and then simmer for 1–2 hours.
Season to taste. If your tomatoes are really acidic, add a little sugar. Mash the whole peeled tomatoes as they cook.
Assemble the meatball subs:
Preheat the oven to 425ºF.
Drizzle the inside of each bun with 1 teaspoon of extra virgin olive oil. Toast upside down on a baking sheet at for 5 minutes.
Remove from the oven, add the meatballs and some extra sauce, and a few slices of mozzarella on top.
Turn on the broiler and broil for 3-4 minutes until cheese is bubbly and browned in spots.