In a slow cooker, combine onion, garlic, broth, wine, and sundried tomato oil; season with salt.
Arrange a rack in upper third of oven; heat broiler. Using a spoon, transfer meatballs and several spoonfuls of broth to a foil-lined baking sheet.
Add gnocchi and remaining ½ cup tomatoes to slow cooker and stir to combine. Cover and cook on High until gnocchi is tender, about 20 minutes.
Meanwhile, broil meatballs, watching closely and turning halfway through, until golden brown in spots, 6 to 8 minutes.
Stir spinach, cream, and ¼ cup Parmesan into slow cooker. Return meatballs to slow cooker. Cover and cook until spinach wilts, about 5 minutes more.
Divide gnocchi and meatballs among bowls. Top with more Parmesan.
