Carefully pop off all the stems off the mushrooms and discard extra membrane if you need to so you can stuff the caps easier.
Quickly rinse off the caps with cold running water and then dry them with a paper towel.
Set mushroom caps aside and dice mushroom stems and onion.
Prepare the Filling:
Preheat a cooking pan over medium heat and add olive oil. Sauté diced onions and mushroom stems first until soft and starts to get golden brown.
Add the sausage to the pan. If you got sausage in the casing, take off the casing and break it up. Cook, breaking up all the clumps, until sausage is done.
Take it out of the pan and let it cool in a mixing bowl for a few minutes.
Mix in all remaining ingredients for the filling until evenly incorporated throughout.
Stuffing Mushrooms:
Stuff each mushroom cap generously, so there is a little dome over the mushroom as well.
Top off each stuffed mushroom with some shredded cheese and place them onto a greased grill topper tray.
Smoking Stuffed Mushrooms:
Prepare the smoker according to the manufacturer’s recommendation and preheat it to 225°-250° and maintain.
Make sure to smoke mushrooms on a grill basket with fine mesh or a grilling topper tray. Mushrooms are too small to place directly onto the smoker’s grate.
Once the smoker is at the temperature, place the tray with mushrooms onto the smoker, close the lid, and smoke for 40-50 minutes.