Actually, the raw mushrooms support my water premise. They leeched water as they baked, which helped with the melt. On one of your future pies, try a water mist. You can also mist water on the cheese prior to topping, but it makes it clump a bit.
How close is the steel to the broiler? If the broiler is very close- such as less than 4", then it tends to freckle rather than evenly tan and bubble the cheese. Is it an extra powerful broiler? It's not IR or anything, right?
That's a little more cheese than I use, but it's still within the parameters.
It's not IR. I believe the broiler is 18000 BTU. My steel is about 5 inches from broiler. I will try misting with water next bake. I might try the broiler on low instead of high next time. Not sure if it will kick on if I put it on low after an hour of 550 preheat though.