This was another I just came up with on my own...
I made meatballs (ground beef, breadcrumbs, raw soy sauce, balsamic vinegar, green onion, mushroom stems) and seared them quickly in a pan.
I preheated the oven to 350F. I took the stems out of some mini portabello mushrooms and placed the caps in a muffin tin.
I put a meatball over each mushroom cap and put it all in the oven. Cook until the meatballs are cooked through.