Preheat oven to 400° and line a large, rimmed baking sheet with parchment.
In a large bowl, add beef, onion, Panko, garlic, parsley, and egg. Season generously with salt and pepper. Mix ingredients together with a rubber spatula or your hands until well combined, then shape into 1” balls. Place onto baking sheet.
Bake until meatballs for 13 minutes, then to each meatball, add a piece of cheese and olive to create “eyeballs.” Bake until cheese is melty, 2 minutes more.
Meanwhile, in a large pot of salted, boiling water, cook spaghetti according to package directions until al dente. Drain and transfer to platter.
Pour warm marinara sauce over spaghetti, then scatter with meatballs.