Melt butter with olive oil in a large saucepan or skillet.
Add onions and garlic and sautee for about 3 minutes, stirring often.
Add meatballs and brown on all sides.
Add tortellini, crushed tomatoes, water, oregano and Italian seasoning. Bring to a boil, reduce heat to medium low and simmer for about 12 to 15 minutes OR until tortellini is cooked.
Stir in ½ cup of mozzarella cheese.
Sprinkle remaining cheese on top. Cover skillet and let simmer for 2 to 3 minutes or until the cheese is melted.
Remove from heat and let stand for 10 minutes before serving.
