Melt butter, add onion and red pepper and saute until just softened.
Add flour and stir to incorporate the flour with the butter/onion mixture. Slowly add the cream, stirring constantly until all cream has been added. Cook until thick and smooth. Add crabmeat, raw shrimp, mushrooms and sherry to the sauce and cook until shrimp are pink through.
Stir in the grated cheeses. Let simmer until reduced and thickened a little more.
Line a deep dish pie plate (or any other type of ovenproof dish) with the pastry (keep the parchment paper and put that down first so the pastry won’t burn to the bottom).
Add the crab/shrimp mixture. Put another sheet of pastry (no parchment) on the top, pinch edges together. Cut vent holes in the top and brush with a little melted butter. Sprinkle with more cheese if you wish.
Bake at 350 for about 30 minutes or until the pastry is puffed and golden.