Thinly slice the shallots, set aside.
Prepare and slice the leeks. Only using the white and light green parts. Cut off the dark green tops. Peel off the tough outer leaves. Starting from the bottom and leaving the root intact, slice to quarter, then turn and cut each leek into about 1/4 inch slices. Rinse if needed.
In a large, deep skillet over medium-high heat add olive oil and add the sliced shallots and leeks. Cook for about 5 minutes until softened, being careful not to let them burn.
Add the chicken and cook another 5 minutes
Add the garlic and cook another minute or so.
Pour in the wine and cook until reduced by about two-thirds.
Add the broth and continue to cook down until reduced by half.
Reduce the heat to medium.
Stir in the sour cream and tarragon and simmer for another 4-5 minutes.
Add salt and pepper, taste and adjust as needed.
Transfer filling to a 9x13 casserole dish.
Place puff pastry on counter, overlaping the 2 sheets about an inch in the middle.
Roll over the seem to connect the two sheets.
Using a pastry brush, lightly brush the sides of the casserole pan with water.
Lay prepared puff pastry over the top. Crimping the edges around the pan.
Poke pastry with fork, or add a few slits with a knife.
Brush with egg wash and sprinkle with flaky salt.
Place in 425° oven for 30-35 minutes, or until golden brown.