For the homemade puff pastry crust: Combine the flour and 1/2 teaspoon salt in a food processor. Pulse a few times. Add the cold butter and pulse until the bits of butter are the size of peas. Add the cold water 1 tablespoon at a time, pulsing in between, until the dough comes together. It will be a bit lumpy.
Shape the dough into a rectangle, then wrap it in plastic and refrigerate for at least 30 minutes. Preheat the oven to 400 degrees F.
Place the dough on a floured work surface and roll out to a 8x20-inch rectangle. Trim the rough edges. Then cut down the center lengthwise. Cut each strip into 5 pieces, creating ten 4-inch squares of dough. Fold each square and place them into muffin tins. Press the centers down to the bottom and press the folds into the sides. Prick the bottoms of the shells several times with a fork. Bake the shells for 15 minutes. Then cool a little.
In the meantime, combine the cream and eggs in a medium sized bowl. Whisk until smooth. Then stir in the crabmeat, tarragon, shallot, nutmeg, cayenne pepper, and remaining 1/4 teaspoon salt.
Scoop the crab mixture into each shell evenly. Bake for 15-20 minutes until the filling is puffed and the edges are golden. Serve warm.