Cut the pastry sheets into twelve 10 cm/4 inch squares. Brush with the melted butter, and line 4 individual fluted brioche tins in 3 overlapping layers. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for
5 minutes. Remove the pastry cases from the oven, leaving the oven on and reducing the heat to 190°C (375°F), Gas Mark 5.
Mix together the crab sticks and prawns. Halve the avocado, remove the stone, scoop out the flesh and dice. Toss the avocado in the lemon juice and stir into the fish mixture. Spoon into the pastry cases.
Blend the cornflour with the milk and stir into the yogurt. Stir in the thyme, garlic, salt and pepper then pour on to the filling. Return the seafood tarts to the oven and cook for 15—20 minutes. Serve hot, garnished with parsley sprigs, if liked.