In a large mixing bowl, combine ground beef, breadcrumbs, egg, salt, and pepper. Use your hands to mix until just blended.

Take a scoop of the meat mixture, flatten it, place a cube of cheese in the center, and roll into a ball.

Preheat oven to 375°F (190°C) and bake meatballs for 25-30 minutes until golden brown.

Boil salted water, add spaghetti, and cook until al dente, about 8-10 minutes. Drain and keep warm.

In a serving dish, layer spaghetti, top with meatballs, and spoon marinara sauce over them.