For the fresh mozzarella: Prepare a bowl of salted warm water and a separate bowl of salted ice water. In the salted warm water, soften the mozzarella curd and pull and shape into a ball. Drop into the salted ice bath to cool. Set aside and pat dry. Slice and let dry on a paper-towel-lined plate.
For the tomato sauce: Heat the oil in a medium saucepan over medium heat. Add the onions and cook until soft, about 4 minutes. Add the garlic and red pepper flakes and cook for 30 seconds. Increase the heat to high and add the tomatoes and oregano if using. Season with salt and pepper. Cook, stirring occasionally, until slightly thickened, about 15 minutes. Taste for seasoning (adding a pinch of sugar if needed) and stir in the basil.
