One of my favorites too, but I don't bake it that long? Usually about 15 minutes is good.

I also add a bit of butter to the dish to cook the mushroom and give more flavor.

There usually is some liquid in the dish from the mushroom but you can pour it off (though it's tasty as well).

There's a brand of tomato sauce in a box that I can get at the supermarket here called Pomi, it tastes really fresh and doesn't have additives. I use it for the portabella pizzas all the time. It is liquid-y but still works well.