Preheat the oven to 400. Brush both sides of the mushroom caps with oil, sprinkle with salt and roast, gill side up, for 10 minutes. Flip and roast until the moisture has cooked off and the mushrooms are mostly cooked through, and additional 8-10 minutes.
While the mushrooms are cooking, prepare the stuffing. Heat a skillet over medium-high. Add the sausage and cook, breaking up the pieces with a wooden spoon, until browned, 6-8 minutes.
Add the leeks and celery to the sausage and saute until soft, 3-5 minutes. Add the garlic and saute until fragrant, 1 minute.
Deglaze the pan with the apple cider vinegar, then add the spinach to the pan. Saute, stirring continuously, until the spinach is wilted.
Remove from heat and fold in 1/2 c. parmesan cheese, pecans, and parmesan. Stuff each mushroom with 1/4 of the stuffing (more like mounding on top, less like "stuffing"). Top each mushroom with an additional one tablespoon of cheese each, then transfer to a baking sheet and place until the broiler until the stuffing is bubbling and the cheese is melted. Serve immediately.