Place in a zippy bag for an airtight container and keep on the counter overnight, or refrigerated for up to 4 days.
Mix the tomato sauce with Italian seasoning, pepper, oregano, sugar, and salt. Fill the tartlets with equal amount of mozzarella, top with equal amounts of tomato sauce mixture, and cover with a pepperoni slice
Bake or air fry at 150 ºC [300 ºF] until the cheese melt. Serve.