Make the Meatballs: In a large bowl, combine the ground beef, ground turkey, egg, breadcrumbs, salt, garlic powder, and pepper. Mix by hand until well combined. Form the mixture into meatballs, about 1¼ to 1½ inches in size (about 2½ tablespoons each). You should get roughly 25–30 meatballs. Place them on a lined baking sheet or plate.
Brown the Meatballs: Heat olive oil in a large, deep skillet or Dutch oven over medium-high heat. Fry the meatballs in two batches, browning all sides (about 5–7 minutes per batch). Use tongs to turn them. Add a little more oil if needed for the second batch. Once browned, transfer meatballs to a plate.
Make the Gravy: Pour out most of the fat from the skillet, but leave the browned bits (they add flavor). Reduce heat to medium. Add the butter and let it melt. Stir in the flour and cook for a few minutes, stirring frequently, until the mixture turns golden (this is your roux). Gradually whisk in the beef broth, making sure the flour fully dissolves. Add Worcestershire sauce, onion powder, and ground sage. Stir well.
Finish Cooking: Return the meatballs to the skillet. Simmer for about 10 minutes, or until meatballs are fully cooked and the gravy thickens to your liking.
Serve: Taste and adjust salt and pepper if needed. Sprinkle with fresh parsley before serving if desired. Serve hot.