Preheat the oven to 425 degrees F and adjust the rack to the middle position. Remove the stems from the mushrooms and finely chop half of the stems and discard the rest. 

Place the ghee, vinegar, 1 teaspoon sea salt and 1/4 teaspoon pepper in a small pot and heat over low until the ghee is melted. Stir to combine. 

Place the mushroom caps in a large bowl and pour the ghee mixture over top and toss to coat the caps. Set aside.

Combine the sausage, scallions, plantain, thyme, garlic, mushroom stems, and remaining salt and pepper in a medium bowl. Using a small spoon, fill the mushroom caps with the sausage mixture and place each cap on a baking sheet lined with parchment paper. Sprinkle the filled mushrooms with parmesan. 

Bake until cooked through and lightly browned on top, about 35 minutes. Let the mushrooms cool for about 10 minutes. Serve. 

To make ahead – The mushrooms can be prepped and stored in an airtight container for up to 24 hours before baking.