Reheating and freezing - I wouldn’t recommend freezing this green bean casserole but you can keep it in the fridge for up to 2-3 days and reheat in the oven at 400-425F for about 10-15 minutes or until the middle is warm.
The fried onions will darken even more after drying, so when they are becoming golden brown during the frying it is time to remove them from oil. Don’t wait until they are dark brown and burned.
Onions - I used red onions but you can use brown onions instead. However the flavor and color will be slightly different. Alternatively, there are ready-made fried onions that you can get from the store.
Flour - I used gluten-free flour from Schar (Universal), basically, any all-purpose gluten-free flour should be fine here as it is used mainly for the sauce and onions and not for baked goods where the type of flour really matters a lot.