Preheat the oven to 400 F.
Wash mushrooms, and remove stems very carefully. Heat olive oil in a large frying pan on med-high heat. Add mushroom caps, sprinkle with a tiny bit of salt, and sauté for 2 minutes each side until softened.
Soften cream cheese in the microwave in a large bowl for about 15 seconds. Add cheeses, lemon juice, salt and pepper, crabmeat, shrimp and green onions, mixing well.
Line a baking sheet with parchment paper. Add mushroom caps, and then spoon filling into mushroom caps, or pipe using a Ziploc bag with the corner cut off.
Cook mushrooms in oven on the lowest oven rack for 15 minutes, then switch oven setting on to broil. Switch oven rack to the top setting and broil tops of the mushrooms for about 2-3 minutes until tops are lightly browned.
Serve right away, or refrigerate for up to 24 hours and then reheat in 350 F oven for 15-20 minutes. Filling can also be frozen up to 3 months to refill fresh mushrooms with!