Preheat oven to 350 degrees F. Line a baking pan with parchment paper or spray well with oil. Place mushrooms on pan and stuff them with the green bean casserole. Bake for 20 minutes or until mushrooms are starting to become brown and giving their juices. Top with a few French's fried onions and bake for another 3 to 5 minutes or until they start to brown a little. Take out and serve.