Lightly grease the slow cooker insert with cooking spray or a tablespoon of oil to prevent sticking.

Place frozen meatballs in an even layer across the bottom.

Nestle halved baby potatoes around and on top of the meatballs.

Scatter the sliced onion over the potatoes and meatballs.

Pour the marinara sauce and beef broth over everything, ensuring it's partially covered.

Sprinkle with salt, pepper, and Italian seasoning if using.

Cover and cook on LOW for 6–8 hours, or on HIGH for 3–4 hours, until potatoes are tender and meatballs are heated through.

Taste and adjust seasoning. If the sauce is too thin, remove the lid, turn the cooker to HIGH for 15–30 minutes to reduce, or stir in a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp cold water) and cook an additional 10–15 minutes.