Place the torn sourdough bread into a large mixing bowl. Pour the buttermilk over the bread and crack in the egg. Use your hands or a fork to mash everything together until the bread has fully absorbed the buttermilk and the mixture resembles a rough paste. Let it sit for two to three minutes to fully hydrate.
Add the salt, onion powder, garlic powder, black pepper, dried parsley, Worcestershire sauce, and finely grated Parmesan to the panade. Mix until evenly combined. If using any optional add-ins, add them now.
Add the ground beef to the bowl last. Using your hands, mix gently until the meat is just combined with the panade. Stop mixing as soon as you no longer see streaks of plain meat. Overmixing develops the proteins and results in tough, dense meatballs.
Cover the bowl tightly and refrigerate for at least one hour, or overnight. This rest period allows the flavors to develop and improves the texture of the finished meatball. The longer rest is recommended when time allows.
When ready to bake, preheat your oven to 430 degrees F. Line a rimmed baking sheet with parchment paper.
Use an ice cream scoop to portion the meat mixture into equal portions of approximately 38 to 40 grams each. An ice cream scoop releases the meat in a rounder shape than a cookie scoop, making the rolling step easier and more consistent.
Roll each portion gently between your palms just enough to smooth the surface. Do not compress the meat. Place each meatball on the prepared baking sheet with space between each one.
Bake for 10 minutes, then remove the pan from the oven and flip each meatball using tongs or a spoon. Return to the oven and bake for an additional 8 to 10 minutes until the exterior is deeply caramelized and the internal temperature reads 160 to 165 degrees F on a meat thermometer.
Allow the meatballs to rest for five minutes before serving. They will continue to carry over cook slightly and the juices will redistribute.