I say, "In my head." It's from years of expiramenting with flavors, imagining what they will taste like in my mind then taking the risk and going for it.
This recipe worked - all the flavors melded together well. I had marinated the sliced mushrooms in a dijon based vinnagrette, so they added a nice acidity that played well with the smooth texture of the goat cheese and sweetness of the onions. Balance was achieved and that's what I am always striving for.