Stuffed mushrooms are infinitely variable and they're great for using up odds and ends of everything you don't really have enough to do anything with in the fridge. Eg: I had kale and shredded cheddar leftover from last night's colcannon, half a red onion I needed to use up, and about half a cup of panko bread crumbs and some bulk pork sausage hanging around. I browned the sausage in a small pan, drained it, and tossed it in a bowl. I squeezed the excess water out of the kale and chopped it, and added it along with a handful of cheese to the bowl. In went the chopped half a red onion and the bread crumbs, I gave it all a couple tosses with my hands to combine, and then smooshed it all into / onto two big portobello mushroom caps (I pulled the stems out and scraped the black gills out with a spoon - this keeps them from getting really watery and mushy and gross). I set them on a foil-lined cookie sheet lightly rubbed with olive oil, and baked them in a 425 degree oven for 20 minutes.
The panko bread crumbs combined with the cheese made for a really nice crunchy crust but the filling underneath stayed moist. The sausage gave a nice little fatty, salty kick to the greens and the onions softened and mellowed nicely. All in all, not bad for just throwing everything in the fridge into a bowl and seeing what happened.