Separate the stems from the mushroom caps. Finely chop the stems then place them in a kitchen towel and wring out excess moisture. Set aside.
Melt 6 tablespoons of the butter over medium heat in a large heavy oven-proof skillet. Add the onion and the diced mushroom stems. Cook until the onion is tender but not browned. Remove from heat and cool slightly.
Preheat oven to 350F/180C.
Place the soft breadcrumbs in a bowl and add the parsley, poultry seasoning, salt, pepper and sausage (if using). Add the entire contents of the skillet (veggies plus the butter they were cooked in). Stir so all the breadcrumbs are coated.
Divide breadcrumb mixture evenly between all the mushroom caps and place them in the skillet you cooked the onions and mushrooms in.
Melt the remaining 2 tablespoons of butter and drizzle over the tops of the stuffed mushrooms.
Bake for 20-30 minutes or until mushrooms being to brown and are crisp on the top.
For EU readers--you may use chestnut mushrooms if you cannot get portobello and US poultry seasoning is roughly a mixture of ground thyme & ground sage, but you could opt for about 1/2 tsp of ground sage to replace the poultry seasoning.