While the mushrooms are draining or earlier, you can start making the stuffing. Heat a skillet over medium high heat and crumble sausage into it. Do not stir for a couple minutes to let it start to brown.
After about 3-4 minutes start to stir and break up the sausage with your wooden spoon. Cook sausage until it is no longer pink.
Remove sausage with a slotted spoon and set aside, reserving about 1 Tbsp of grease if there is any.
If you don't have much grease left, add 1 Tbsp of olive oil to your pan. Now add the onions and mushroom stems to skillet and cook on medium about 5 minutes or until tender.
Stir in garlic and cook another minute.
Add sherry and worcestershire sauce and cook about another 5 minutes, or reduced.
Then add the oregano, pepper and sea salt and cook another minutes, stirring occasionally and reduce heat to low.
Return sausage to skillet, add cream cheese and parmesan to the mushroom/onion mixture and stir till thoroughly combined with no lumps of cheese.
Then remove from heat and add to a medium mixing bowl and let cool slightly so that it's not too hot to fill mushrooms.
Taste your mixture to make sure it's seasoned to your liking here. Once you approve the taste you can add your egg, mix well and start filling the mushrooms.