Step 1: Make the Meatball Mixture

1. In a large mixing bowl, combine the ground beef, ground turkey, breadcrumbs, Parmesan cheese, milk, egg, minced garlic, nutmeg, salt, and pepper.
2. Use clean hands to gently mix everything together until just combined. Be careful not to overmix, as this can make the meatballs dense.
3. Roll the mixture into golf-ball-sized meatballs (about 1.5 inches in diameter) and set aside on a tray or plate.

Step 2: Sear the Meatballs (Optional but Worth It)

1. Heat a tablespoon of olive oil in a large skillet over medium-high heat.
2. Working in batches, sear the meatballs for 1-2 minutes on each side until they develop a golden-brown crust. (Don’t worry if they’re not fully cooked; they’ll finish cooking in the slow cooker.)
3. Transfer the seared meatballs to the slow cooker.

Step 3: Prepare the Gravy

1. In the same skillet, add a little more olive oil if needed, and sauté the onions and mushrooms until softened and slightly caramelized, about 5 minutes.
2. Add the beef broth, Worcestershire sauce, ketchup, and Dijon mustard to the skillet. Stir well, scraping up any browned bits from the bottom of the pan (this adds so much flavor!).
3. Bring the mixture to a simmer, then pour it over the meatballs in the slow cooker.

Step 4: Slow Cook

1. Cover the slow cooker and cook on low for 6 hours or high for 3 hours.
2. About 30 minutes before serving, mix the cornstarch slurry and stir it into the gravy to thicken. Replace the lid and let the slow cooker finish cooking.

Step 5: Serve and Enjoy!

1. Once the meatballs are tender and the gravy is thick and glossy, they’re ready to serve.
2. Garnish with fresh parsley or chives for a pop of color, if desired.