Preheat oven to 375 degrees F. Line a large baking sheet with parchment paper.
Add ground beef, onion, garlic, salt, and pepper to a large pot. Cook over medium-high heat, chopping the meat up with a wooden spoon, until the meat is no longer pink.
Pour in the beef broth and Worcestershire sauce and stir, scraping the bottom of the pan to lift any browned bits. Add the marinara sauce and the Italian seasoning; stir to combine. Cover, reduce heat to low, and simmer for 20 minutes.
Cook the spaghetti al dente according to the package directions. Drain and add it to the pot of sauce. Stir to combine and coat the noodles well. Remove from heat and let rest while you prepare the kaiser rolls.
In a small mixing bowl, combine the melted butter, garlic powder, salt, and parsley.
Slice the tops off the kaiser rolls with a serrated knife. Using your fingers, hollow out the centers, leaving a ¼-inch thick wall. Use all the butter mixture and brush onto the inside of the rolls. Bake for 10 minutes until just lightly toasted.
Add a pinch of shredded mozzarella to each roll. Evenly distribute the spaghetti into each roll and top with the remaining cheese. Bake for 8-10 minutes, until cheese is melted and starting to bubble.