Preheat oven to 350. Wash mushrooms and remove stems (don't discard!). With a spoon, scoop out insides of mushrooms until a nice 'bowl' is created, and place each mushroom in a muffin slot.
Finally chop inside of mushrooms and stems and put in a mixing bowl. Option to omit this step, but I find using the rest of the mushroom ties everything together and doesn't waste as much, but personal preference.
In skillet, saute Italian Sausage (if using a link, slice down the middle to remove casing and cook as ground sausage) into very small pieces. If needed when it cools, chop even finer with a knife until the sausage is about rice size. Add to the mixing bowl with the mushrooms.
Chop down the spinach into very small pieces and add to bowl, with minced garlic, olive oil, seasonings (Italian seasoning, crushed red pepper, salt and pepper), ricotta, mozzarella and about half of the parmesan. Mix well.
Using a tsp, carefully spoon the mixture into each of the mushrooms. You will likely have leftover mixture. See notes at the bottom.
Bake for about 20 minutes, or until the tops are starting to brown. Remove and using two spoons, life each mushroom out of the pan. Sprinkle with additional parmesan cheese and enjoy!
Note: As an appetizer, you can fill these mushrooms more full to use the additional mixture, but run the risk of them spilling over and creating an unpleasant looking dish. You can use the remainder of the mushroom package in another batch without adding more of the mushroom interior to create more for the crowd.As a meal, I made zucchini noodles and mixed in the extra mixture, served with some of the mushrooms on top and a side salad. Delicious and easy!