Heat the sauce: Warm the marinara sauce in a pot over low heat until it reaches a simmer.
Make the meatballs: In a bowl, mix the breadcrumbs, milk, ground beef, oregano, salt, and pepper until well combined. Form into equally sized meatballs using your hands.
Sear the meatballs: Heat a non-stick pan over medium heat. Add the meatballs in batches without crowding and sear until nicely browned (about 3 minutes per side).
Simmer in sauce: Transfer the browned meatballs to the pot with marinara sauce. Partially cover with a lid, stir occasionally, and simmer for 20 minutes.
Cook the pasta: Boil a pot of salted water. Cook the spaghetti according to the package instructions. Drain well.
Assemble the dish: Remove the sauce from the heat. Serve the meatballs over the pasta, ladle sauce over top, and garnish with parmesan cheese.
Make the arakas latheros: In a saucepan, heat olive oil and sauté the onion for 2–3 minutes until soft and translucent.
Add the green peas and stir to coat in oil. Stir in tomato paste for 1 minute, then add water, salt, and pepper.
Bring to a boil, then cover and simmer for about 15 minutes.
Add the fresh dill and continue simmering for 5 more minutes, until the peas are soft and most of the water has evaporated.
Serve: Plate the spaghetti and meatballs with a side of arakas latheros, and white wine.