Small insert to cook in large slow cooker - add water to slow cooker or not?

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I have a huge slow cooker and want to cook a small amount of rice pudding.

So if I put a metal trivet in the slow cooker, then a small Pyrex dish on top with the rice pudding in, do I need to add water to the slow cooker?

Or just foil on the rice pudding then the slow cooker lid but NO water?

Will the slow cooker actually cook something in a dish that’s in the cooker? It’s not an oven.

Yes. I’m just unsure whether I need to add water or not.