Preheat the oven to 400° F with the rack in the center of the oven and place a foil-lined baking sheet onto the rack.
Gather your 'shrooms and wipe the tops clean with a damp cloth and remove the stems and gills with a spoon. Once cleaned, put them in a shallow baking dish.
Use a sharp paring knife to cut a shallow "X" on the top of each 'shroom and brush avocado oil all over the 'shrooms and season the tops and bottoms with salt and pepper.
Place the mushrooms on the preheated baking sheet in the oven, gill side up, and bake for 10 minutes. Flip each mushroom and bake them gill side down for an additional 10 minutes. Remove the tray from the oven and let the mushrooms cool to room temperature.
Increase the oven temperature to broil.
While the mushrooms are cooking you can start making the stuffing. Heat the ghee in a large skillet over medium heat and sauté the minced onion (with salt and pepper) until they are soft and translucent.
Add the sausage and a few dashes of Sunny Paris seasoning to the skillet. Cook the meat until it is no longer pink.
Remove the meat mixture to a medium bowl and let it cool to room temperature.
Once the sausage is cooled, add the egg, spinach, coconut flour, salt and pepper and mix to combine.
Transfer the now cooled roasted mushrooms onto another baking sheet lined with foil (the original baking sheet will have lots of mushroom liquid on it) and piled the stuffing onto each cap – pressing down to make the stuffing more compact.
Place the tray under the broiler (center rack) for about 5 minutes, rotating halfway during the cooking time.
The stuffing should be evenly browned (but not burned) when the mushrooms are finished.
Top the stuffed mushrooms with marinara sauce and serve immediately!