1. For the filling: Bring a medium pot of salted water to a boil, add onions, and cook over medium heat until soft, 10–15 minutes. Drain; then return onions to pot. Add beef, butter, oil, beef fat, oregano, sugar, flour, peppers, and salt to taste. Cook over medium heat, stirring often, until meat is cooked through but not browned. Adjust seasonings. Transfer to a bowl, let cool, cover, and refrigerate for at least 2 hours or overnight.