Instead of flour and cornstarch, try adding gluten free Idahoan potato flakes until just shy of desired consistency as it thickens a bit more. Also used canned milk in place of cream.
I haven't tried this particular recipe, but I made 1 similar to this- actually this recipe sounds closer w/ my substitutions than the one 1 made.....I also added a pound of fresh mushrooms slice...
Could have used more seasoning and thickness according to my husband. I liked it as it was.