I also amended the recipe a bit. I used an onion and 2 cloves of garlic instead of the leeks. I also did not cook off all the mushroom liquid or sherry. Instead of mixing the flour to the mushrooms, I blended it into the milk and added this to the mushroom mixture, cooking until thickened. I found this created a moist but not runny casserole. Finally, I mixed everything together instead of layering it. It was very tasty. I used a gluten free flour mix and it worked perfect, too.