Preheat your oven to 400 F/ 200 C and line a baking sheet with parchment paper. Remove the mushroom stems and place the caps on the baking sheet. Season with salt and pepper and set the baking sheet aside.
Dice the stems, finely chop the onions and mince the garlic. Heat the butter in a non-stick pan over medium-high heat. Add the mushrooms, onion, and garlic to the pan.
Cook for 4-5 minutes, stirring frequently. It’s ready when all the veggies are soft, and all the liquid has evaporated. Set the pan aside.
Add the cream cheese, cooked mushroom mix, 4 tbsp grated parmesan, and crushed red peppers to a bowl. Finely dice the cornichon and crab and add them to the bowl. Season with salt and pepper and mix to combine.
Add 1 tbsp of filling to each mushroom. Top with the remaining parmesan cheese. Transfer the baking sheet to the oven.
Bake for 20 minutes or until golden all around. Transfer the stuffed mushrooms to a serving plate and garnish with parsley and lemon wedges. Serve immediately.