My version of this recipe started with a recipe in a magazine. The original recipe called for carrots, beans, peas, and squash artistically arranged in an ovenproof bowl (as the shell for the mashed potatoes). That seemed so time consuming. I decided to limit the veggie shell to sliced carrots. You can use any combination of vegetables you like for the shell.
I was desperate to find a way to get my DH to eat broccoli and cauliflower. I found that he would eat it with cheese sauce on top. Now that he's accustomed to eating those veggies, I don't have to resort to cheese sauce any more, unless we're in the mood for it.
The broccoli in the stuffing is a nice change from the usual crab or sausage you find in many recipes. Do make the effort to add roasted bell pepper to the tops as it makes the finished mushrooms look really special. I found this recipe a few years ago in a magazine, but only tried it recently. Now it's my favorite mushroom recipe.