Combine ground beef, ground pork, spring onions, fresh oregano, 1 clove garlic, Worcestershire sauce, parsley, salt, and black pepper in a bowl. Mix well. Add 1 1/3 cups bread crumbs gradually. Form mixture into balls.
Place remaining 1/3 cup bread crumbs in a bowl. Dip meatballs into bread crumbs and transfer to a tray. Cover with plastic wrap and refrigerate, at least 30 minutes.
Heat oil in a large skillet over low heat. Cook meatballs in batches, stirring and turning occasionally, until evenly brown and no longer pink in the center, 5 to 7 minutes. An instant-read thermometer inserted into a meatball should read at least 160 degrees F (70 degrees C).
Heat 2 tablespoons oil in a skillet. Add pancetta; fry until brown but not crisp, about 4 minutes. Add onion and the remaining garlic; cook and stir until fragrant, about 1 minute. Add red bell pepper, carrot, and celery. Fry, stirring occasionally, until softened, about 3 minutes. Add oregano, basil, coriander, and black pepper; cook and stir for 1 minute.
Pour wine into the pancetta mixture and stir to reduce. Stir tomato puree in quickly to avoid burning. Add plum tomatoes; stir to break down. Bring to a boil, reduce heat, and simmer, stirring occasionally, until sauce thickens considerably, 1 to 1 1/2 hours.
Blend sauce using an immersion blender. Add meatballs to the sauce; cook until heated through, about 10 minutes more.
We have determined the nutritional value of oil for frying based on a retention value of 10% after cooking. Amount will vary depending on cooking time and temperature, ingredient density, and specific type of oil used.