In a large mixing bowl, combine the ground turkey, breadcrumbs, Parmesan cheese, onion, garlic, parsley, egg, salt, black pepper, and oregano. Mix gently until just combined. Do not over mix.
Shape the mixture into 20-24 meatballs, about 1½ inches in diameter.
Pour 1 jar of marinara sauce into the bottom of a 6-quart slow cooker.
Arrange the meatballs in a single layer over the sauce. If needed, slightly overlap them, but avoid packing them too tightly.
Pour the remaining jar of marinara sauce evenly over the meatballs, making sure they are well covered.
Cover and cook on Low for 4-5 hours or until the meatballs are cooked through and reach an internal temperature of 165°F.
Gently stir the meatballs to coat them in the sauce.
Garnish with chopped fresh basil, if desired, and serve warm over pasta, mashed potatoes, or with crusty bread.
Notes
Refrigerate leftovers in an airtight container for up to 4 days.
Freeze the cooked meatballs and sauce in a freezer-safe container for up to 3 months.
Plain or Italian-seasoned breadcrumbs both work well. Italian-seasoned breadcrumbs add extra flavor.
Mix the meatball ingredients just until combined to keep the meatballs tender rather than dense.