Step 1
Preheat the oven to 425 degrees F (220 degrees C).
Step 2
Cut pastry into 6 rectangles and place on a baking sheet.
Step 3
Bake in the preheated oven until puffed and golden 15 to 20 minutes. Remove from the oven; leave the oven on.
Step 4
Meanwhile cut mahi mahi into 1 inch pieces. Season with a pinch of salt and pepper.
Step 5
Heat olive oil in a large pot or Dutch oven over medium heat. Add potato celery and carrot. Cook stirring occasionally until just tender about 8 minutes. Add leeks and let soften about 3 minutes. Add garlic and cook until fragrant about 30 seconds. Add butter.
Step 6
When butter has melted add flour and whisk for 1 minute. Slowly whisk in milk and water then add 1 teaspoon salt and 1/2 teaspoon pepper. Bring to a boil.
Step 7
Reduce heat to a simmer and cook until mixture thickens slightly 3 to 4 minutes. Whisk in sour cream lemon juice and Dijon. Add fish and cook over low heat until fish flakes easily with a fork 7 to 8 minutes. Stir in parsley and chives and remove from the heat.
Step 8
Transfer fish mixture to a 2-quart pastry dish. Place pastry rectangles on top and brush each one with beaten egg.
Step 9
Return to the oven and bake until pastry is puffed and golden brown and filling is bubbly about 10 minutes. Let stand for 5 minutes before serving.