Prepared the recipe as pretty nearly as directed and turned out quite good, but in the future I would change the following:
1) Onion Filling - 3 Lg. Sweet onions were not enough, I'd add another one for a total of four. Also, I used 2 large portabello caps and would increase to 3 next time.
2) Spinach & Cheese Filling: Add a couple cloves of garlic to the mixture and a 1/4 cup parmesan. I found the mixture a bit too bland.
3) White Sauce and Topping: I would double the quantity of white sauce to make sure there was plenty to top the dish. Sprinkled extra gorgonzola on top.
Also, considering using italian soy sausage so that it stays meatless, but adds a little kick.