I have made stuffed bell peppers, stuffed artichokes, and my latest stuffed onions. I love sauteed onions so baking a whole onion sounded like an appealing idea to me. I found the basic recipe from one of my twitter friends: The Naked Kitchen. I roasted my onions in garlic infused olive order and my "stuffed" mixture was slightly different. You can go the vegetarian route and keep it stuffed with just mushrooms, spinach, and onion, and serve as a side dish, or you can add some sliced, cubed pieces of sausage and make it your entree. I recommend the Aidell's brand. I mixed my sauteed mushrooms, spinach, and sausage mixture with a little almond flour, about 3 tbsp to help hold it all together.
You may end up with more stuffing than you need, like I did, so I actually separated some of the mixture and just put it in tubberware minus the almond flour to eat later. You could add some cheese, but it's really not needed. Just season accordingly and you should end up with a very nice dish.