Pour 1 tablespoon olive oil into a pan set to medium heat. Add the onion and cook until translucent. Then transfer the onions to a bowl and cool.
Combine the milk and panko bread crumbs. Add the egg and mix well. Add the ground beef, onions, salt and a pinch of pepper and knead until all ingredients are fully incorporated. Divide the mixture into 24 portions. Place a cube of cheese in the center of each piece and fold the meat around the cheese to seal it in.
Set a frying pan to medium heat and add 1 tablespoon olive oil. Place the meatballs in the pan and brown on all sides. Add the canned tomatoes and cherry tomatoes and mix to coat the meatballs. Place a lid on the pan and cook until the meatballs are fully cooked. Remove the meatballs and reduce the sauce to a simmer. Simmer until the sauce thickens to your desired level. Season with salt and pepper, then add a drizzle of olive oil. Note: the cherry tomatoes sweeten the sauce, so don't be hesitant when seasoning with salt and pepper - I had to add more than I expected.
In the meantime, bring a large pot of salted water to a boil and cook the spaghetti as long as directed on the package.
Twirl the spaghetti onto your plates and spoon the meatballs and sauce on top. Grate some parmesan cheese and parsley on top and enjoy!