Grate the onion using the large holes of a box grater over a large bowl. You should end up with about ¼ cup.
Chop the spinach and basil into very small pieces. Alternatively you can add the onion, basil, and spinach to a food processor and pulse until finely chopped but not pureed.
To the same bowl, add the nutritional yeast, garlic powder, and salt, stirring to combine. Add the turkey and stir again, until all the ingredients are well incorporated.
Add the olive oil to the base of a 6-quart slow cooker, spreading it around until the bottom is well coated.
Using a small, 1.6-inch cookie scoop, scoop the turkey mixture into a single layer into the base of the slow cooker. Gentle scoop any remaining mixture on top, creating a second layer. If you don’t have a cookie scoop, you can use a large soup spoon instead. Ground turkey is very sticky, so trying to roll them into balls by hand will likely result in a mess, so use the scoop or spoon method for best results.
Pour the marinara sauce over the meatballs. Do not stir.
Cover and cook on high for 4 hours or low for 6-8. Uncover and gently stir to separate the meatballs.
Serve over pasta, spaghetti squash, or with crusty bread, topped with fresh basil, parmesan (traditional or dairy-free), and red pepper flakes if desired.