Make the filling. In a heavy-bottomed pot or skillet over medium high heat, heat the oil.
Add the sausage and cook, breaking the chunks apart with your spatula or spoon, until browned, 1 to 2 minutes per side. Remove meat from the pan, reserving the oil.
Add the onions, and cook, stirring, until soft, 3 minutes.
Return the meat to the pot, add the garlic, basil, oregano, and ground black pepper and cook, stirring, until fragrant, 1 to 2 minutes.
Add the pasta sauce, tomato paste, juice, and sugar, and stir to combine. Break the tomatoes into small chunks using a wooden spoon.
Bring to a boil. Reduce the heat to medium low and simmer, stirring occasionally, until most of the liquid has evaporated and sauce has thickened, 20 to 25 minutes.
Add the salt and red pepper flakes, and stir to combine. Remove from heat.
Spread the filling in a tray and let cool, about 30 minutes. Divide the filling to roughly 16 equal-sized portions.