Chloe's Vegan Italian Kitchen with small modifications: the addition of a bit of minced seitan and a change in spices. I'm reproducing it here, but strongly advise all of you to buy the book, which has many more wonderful recipes!
2 tbsp olive oil
1 cup cashews
4 garlic cloves
1 cup cooked brown rice
2 cups cooked lentils
1/2 cup seitan
1/4 cup breadcrumbs
1/2 cup vegetable broth (we used the broth in which the seitan was stored)
1 tbsp Herbs de Provence
10 portobello mushrooms
Preheat oven to 350 and drizzle baking sheet with olive oil. Remove gills and stems from mushrooms and thinly mince. Heat up olive oil in a pan, slice up the onion and fry them up with the cashews until translucent. Add garlic continue sauteeing a few more minutes.
Transfer onion mixture into bowl and add rice, lentils, chopped up seitan, broth, breadcrumbs, and spices.
Brush both sides of each mushroom with a bit of oil, place in one layer on baking sheet, and generously spoon stuffing on top of each. Place a thin slice of tomato on each mushroom. Place baking sheet in oven and bake for about 30 minutes.
We had four leftover mushrooms, which we used to make mini-burgers for the next day. Stay tuned!