Start by preheating the oven to 375 degrees.
Make the turkey meatball mixture. Add the ground turkey, Italian sausage, breadcrumbs, egg, milk, Worcestershire sauce, parmesan cheese, and seasonings to a large bowl.
Gently mix until all ingredients are combined.
Place foil or parchment paper on a large-rimmed baking sheet. I like to give it a quick spray with cooking spray, so the meatballs don't stick. You can use an oiled or sprayed baking sheet if you don't have foil or parchment paper.
Use a medium scoop or ice cream scoop to portion out the meat mixture. Roll each portion into a ball and place it on the baking sheet.
Continue with the remaining meatball mixture. You'll end up with about 20 - 24 meatballs. The meatball mixture is on the wet side, so it's helpful to have a small cup of water available as you roll the meatballs.
Bake meatballs for 20 minutes.
Place the meatballs into the bottom of the slow cooker insert and then pour the marinara sauce over the meatballs.
Gently stir until the sauce coats the meatballs.
Cook on low for 4-6 hours.