I love mushrooms stuffed with a sauteed mixture of veggies & tahini. I usually do it with baby portabellas & scoop out the inner 'meat' to add to the saute, so if doing it with a portabella, perhaps use some extra mushrooms chopped up in the saute to give it more body? I usually use chopped garlic, chopped onion, shredded carrots, and chopped mushrooms innards. Saute in oil of your choice, with a little salt and pepper...a shake of nutritional yeast if you want (if adding, add it near the end) --saute till soft, turn off heat, and add a big blop of tahini & stir briskly into the hot saute. It will get sticky and cohesive (cohesiveness will last through baking and bind so the veggies are not just falling apart out of it). Can add jalepenos (I use the canned ones, just chopped up), or some parm or cheddar cheese, add to the portebella, do in the oven at about 350 in a pan covered with foil till mushroom is soft, remove foil and cook uncovered until the toppings get crispy across the top (another 5-15 mins, depending on how often you open the oven to check and let the heat out LOL). I do this in stuffed peppers, too. Yum!
(side note: I like the versitility of these. This is also my basic 'recipe' for stuffed peppers, too. They are great without any cheese (in which case, they would be nut/grain/dairy/soy/corn freeeeeee!...I have a veggie/allergy kid, so this excites me haha....), though I like them with cheese and jalepenos, for the non-vegan crowd.

Also, (I hope this is not distasteful to mention here, I know we veggies are all across the board in terms of whether our family members eat meat or not and whether we are ok fixing it for them...) my DH and older son eat <meat>, and these mushrooms are very easy to add some of that (will spare details;if anyone needs details, PM me) to for them.)
ETA:
I also make a yummy lentil dish that we do for 'mock stuffing' (we have a wheat free kiddo): saute chopped garlic, onion, celery, add lentils and cook according to directions, add s & p and dried sage, sooo yummy. They are very thanksgiving-y, would prob be good as stuffing in a mushroom too. You could even sprinkle some roasted nuts, or dried cranberries atop it for an extra festiveness.
