Heat olive oil over medium heat in a large skillet. Sauté leeks until translucent and fragrant, about 5 minutes. Add carrot, celery, fennel and mushrooms. Sauté until carrots are nearly done, about 8 minutes.
Add wine to the vegetables, cook until reduced by half, about 4 minutes. Add mushroom stock, corn starch, salt, and pepper. Bring to a low boil, stirring until thickened, about 1 minute. Gently fold in peas, potatoes, thyme and tarragon. Taste and adjust seasoning if desired.
Line a rimmed baking sheet with parchment paper.
Roll out pie crusts on a lightly floured board. Using a glass or 3-inch biscuit cutter, cut about 10 circles from each crust, re-rolling scraps if necessary.
Spoon 1 Tbsp veggie filling on half of the pastry rounds, to within ½ inch of the edge. Using your finger, wet dough edges with water. Cover with another pastry round, crimp edges with a fork. Transfer to parchment lined baking sheet.
Cut 2 slits into the top of each pie, brush with egg wash. Bake 10-12 minutes, or until golden brown.
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