Heat oil over medium-high heat in a medium saucepan. Add onion and cook 3 minutes. Add garlic and cook 2 more minutes.
Add remaining ingredients and simmer for 15 minutes.
To make meatballs, preheat oven to 400 degrees. Combine all ingredients in a large bowl, mixing well.
Shape into 24 meatballs and place on a lightly greased baking sheet.
Bake for 20 to 25 minutes, or until cooked through. Let meatballs cool for 15 minutes before slicing. (Or you can refrigerate them at this point and use them in the next 48 hours.)
The flatbread will be more pliable and less likely to tear if you microwave it. Stack them on a plate and cover them with a damp paper towel. Microwave for 30 to 40 seconds.
Lay 2 slices of mozzarella on one end of flatbread.
Cut meatballs in half and arrange them on top of cheese. Spoon desired amount of tomato sauce on top. Place 2 more slices on mozzarella over the sauce and fold the other half of the bread over.
Using the butter to coat a nonstick pan or a panini press, cook each sandwich on both sides until golden and the cheese is melted.