You can even use xanthum gum, and corn starch - the issues with using flour and corn starch however - is that you will taste them more. They get better with cooking.
You can use cheeses to thicken, or some heavy cream.
I have heard of flour but never Instant mashed. Great Idea. I will have to try some next time I make a soup. All my soups have been Tomato based thus far. Be making alot of soups me thinks come winter time!