For the pie crust, place flour and salt in a food processor and pulse to combine.
Add the cold butter and pulse just a few times until the mixture is crumbly with pea-sized pieces of butter remaining intact.
Add 2 tablespoons of ice water and pulse until the dough starts to stick together. Add more ice water 1 tablespoon at a time only if it looks too dry. Don't add too much water or it will get tough. It's ready when it sticks together when pinched.
Transfer the dough to a work surface and quickly shape it into a flattened ball without kneading it. Wrap the ball in plastic wrap and chill in the refrigerator for an hour.
In the meantime, start making the filling by melting the butter in a skillet on medium.
Add the shallots, garlic, and celery to the pan and sauté for about 5 minutes, stirring occasionally.
Sprinkle the flour over the vegetables and stir well to mix evenly. Cook for 2-3 minutes to thicken while stirring occasionally.
Pour in the wine, bring the mixture to a boil, and then lower it to a rapid simmer for about a minute, stirring occasionally.
Add the fish stock, salt, and pepper, bring the mixture back to a rapid simmer, and let it cook for about 5 minutes, stirring occasionally.
Add the cream and stir to mix well.
Gently place the shrimp, scallops, and crab in the skillet along with the thawed vegetables and parsley and cook for 2-3 minutes, stirring occasionally. Remove from the heat.
Preheat the oven to 400 F.
Cut the ball of dough into 4 equal pieces, place 1 on a lightly floured surface, and roll it out into a flattened disc about ⅛ inch thick and large enough to cover the top of your ramekin plus some overhang. Repeat with the 3 other pieces.
Ladle filling into each ramekin and moisten the edges of the ramekins with water to help the crust adhere to them.
Center the dough discs over the filling, fold the extra dough under, finger press the edges, and crimp them with a fork to seal.
Cut slits in the top of the dough.
Place the ramekins on a rimmed baking sheet and bake for 30-40 minutes until the crust becomes golden brown and the inside is hot and bubbly.