Mo, I've used a glass 10x12 Pyrex or a large deep 8" casserole. Of course,
you will need to judge the amount remaining after you're done testing...and testing....and testing.
When bringing it in for pot-luck luncheons at work, I label it as "Mashed Potatoes with Caramelized Onions"---since some folks hesitated the first year when it was called "Russian Potato Casserole". They wanted to know what the "brown things were"...ie, caramelized onions.
(I work with a LOT of recently-graduated college-age men.)
Get asked for the recipe every single time. By the women.
The texture will change a bit if you reheat in microwave. Thankfully, the taste is still a killer. Oh, and confirm your potluck dish actually FITS in the microwave at work or you will be forced to scoop out and reheat in portions. Please don't ask how I know to put this warning here for you.