In Reply to: Need help with this recipe, please: Vidalia Onion posted by and Rice Casserole. sm on Mon, May 10 2021 at 11:00 AM CDT:
This is why it's always better to give ingredients by weight. Fortunately, this isn't the kind of recipe where you need to be very prices.
So, very roughly, 1 cup of chopped onions weighs about 160 g according to the USDA, so you would need around 2-1/2 lb (1120 g) of onions for 7 cups chopped. That should be about 3 large sweet onions, but it all depends on their size. Simplest is to weigh what you've got.
I see no reason why you couldn't freeze and then microwave any leftovers.