Defrost the meatballs by placing the sealed bag of meatballs in a bowl of warm tap water. Be careful not to allow water into the bag.
Preheat the oven to 400 degrees.
Slice the bell pepper and half of the onion, and then water-sautee over medium-high heat. Start with a tablespoon or so of water, and stir in the sliced veggies. Allow them to cook for a few minutes, then deglaze the pan with another tablespoon or two of water while stirring. Be careful to deglaze whenever the pan gets dry, or you will burn the veggies. Repeat until they begin to caramelize (about 10 minutes).
Meanwhile, Make the Quick Blitz Marinara: In your blender jar, add the tomatoes first, then the other half of the onion, the garlic, the chili flakes, and the fresh basil.
Pulse for 20-30 seconds, until well combined and the desired texture is achieved.
Pour the mixture into a saucepan on medium-low heat and add the meatballs. Simmer until the sauce is heated through meatballs are warm inside. About 10 minutes.
While that simmers, cut a wedge out of the top of both of your sub rolls or italian bread loafs. If the bread is pre-sliced, you can skip this step and just open the bread and place it on a baking sheet lined with foil.
Toast the rolls for about 2 minutes in the oven and then take it out.
Assemble your sandwiches. On each of your toasted rolls, add a layer of peppers and onions, then 3-4 of your simmered meatballs, spoon over a little marinara sauce, and then top with 1 or 2 slices of mozarella cheese. Put your sandwiches back in the oven for 3-4 minutes, or until the cheese begins to melt. You can broil it for the last minute to toast the cheese to a golden-brown, if desired.