1. Prepare the Meatball Mixture: In a large mixing bowl, combine the ground turkey, finely chopped fresh spinach, breadcrumbs, chopped yellow onion, egg, minced garlic, ground ginger, salt, and black pepper. Use your hands to gently mix until all ingredients are just combined. Be careful not to overmix, as this can lead to tough meatballs.

2. Form the Meatballs: Roll the mixture into 1-inch to 1.5-inch meatballs. You should get about 25-30 meatballs, depending on their size. Place them carefully on a plate or baking sheet as you form them.

3. Arrange in the Slow Cooker: Lightly grease the inside of your slow cooker insert. Carefully place the formed meatballs in a single layer at the bottom of the slow cooker. If you have too many to fit in one layer, you can gently layer them, but try to keep them as spread out as possible.

4. Whisk Together the Sauce: In a separate medium bowl, whisk together the undrained crushed pineapple, teriyaki sauce, and brown sugar until the sugar is fully dissolved.

5. Pour Sauce Over Meatballs: Pour the prepared teriyaki pineapple sauce evenly over the meatballs in the slow cooker, ensuring they are well coated.

6. Slow Cook to Perfection: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours. The meatballs should be cooked through, and the sauce will be bubbly and flavorful.

7. Thicken the Sauce (Optional): About 30 minutes before serving, in a small bowl, whisk together the cornstarch and cold water to create a slurry. Stir the slurry into the slow cooker with the meatballs. Cover and continue to cook until the sauce has thickened to your desired consistency.

8. Serve Hot: Once the meatballs are cooked and the sauce is thickened, they are ready to serve.