Carefully snap the stems off of each mushroom. Put the mushroom caps on a foil-lined cookie sheet, and place the stems on a cutting board.
Once all of the stems are removed, dice them up and put in a skillet on your stove over medium heat. Add a tablespoon of butter, and let the stems cook for about five minutes.
Add a clove of minced garlic to the mushrooms, along with a stalk or two of chopped green onion and continue to cook for another minute or two. When the mushroom stems and garlic are done cooking remove the skillet from the stove, and put the mushrooms and garlic in a small bowl and place in your freezer. You want to cool this down before adding to your mushroom caps.
In a small bowl combine the cream cheese, grated parmesan cheese, and shredded cheddar cheese.
When the cooked mushroom stems and garlic has cooled down add that to the cheese mixture. Sprinkle in a few red pepper flakes and some salt and pepper, and then scoop a little bit of the mixture into each of the mushroom caps. Your caps might be heaping over with the cheese mixture, and that's ok.
Brush each of the mushrooms with melted butter and lemon juice.
Turn on your Traeger to smoke. When the smoke is rolling turn the temp up to 350 and put the cookie sheet in the smoker.
Let the mushrooms cook for about 20 minutes and then remove from the smoker. Let them cool for a few minutes, and enjoy!