I don't have measurements, but here are a couple of ways I do them in the oven:
saute chopped onions, celery, carrots and mushrooms.
cool and add a bit of breadcrumbs, egg and percorino romano cheese (or any cheese you like)
I scape out the gills of the portabella, put in a piece of provolone (again or whatever you like), then stuff with the above mixture.
Coat the out side of the cap with just a bit with oil, put a bit of wine and chicken stock in a baking dish and bake for about 20 minutes at 350º.
The other way I do them is to make a crab cake mixture and stuff the mushrooms with it. I do the same thing with the wine and stock.
I baste them a few times during cooking.
Sometimes I serve them with oven roasted bliss poatoes and a nice salad for a meatless meal.