Combine ground beef, onion, panko, eggs, salt, pepper, and parsley in a bowl and stir to combine.
Add butter to melt in a large, oven-safe skillet over medium-high heat.
Once the skillet is hot, form your ground beef mixture into meatballs, using 2 tablespoons of meat for each one. Carefully place your meatballs in the skillet, evenly spaced, to ensure even cooking.
Cook the meatballs for 1-2 minutes until browned on each side.
Combine garlic and Worcestershire sauce and add to the browned meatballs.
Stir in the marinara sauce and bring to a boil. Reduce heat and continue to simmer for 5-10 minutes.
Sprinkle cheese on top and put the whole skillet in the oven to bake at 360F for 15-20 minutes.
Serve with fresh chopped herbs, and enjoy :)
Video
Notes
Prepare your meatballs and sauce ahead of time to streamline the cooking process. If you're planning to make a larger batch, you can even freeze the meatballs for later use.
Cover the casserole dish with aluminum foil while baking to prevent the cheese from burning and to ensure the meatballs cook through evenly.
Allow the meatball casserole to rest for a few minutes after baking before serving. This will allow the flavors to meld together and make it easier to slice and serve.