Preheat oven to 425 degrees F. Spray 4-inch ramekins or oven-safe small pots with nonstick spray.
Place a large skillet on the stove over medium heat. Melt the butter.
Once butter has melted, add the onion, carrots, and celery. Cook for 5 minutes, or until the onion starts to turn translucent.
Add in cremini mushrooms and garlic, stirring occasionally so the garlic does not burn. Cook 3 more minutes, until mushrooms are tender and any liquid they give off evaporates.
Stir in flour until all vegetables are coated. Carefully pour in chicken broth and heavy cream. Whisk together until combined. Simmer for 3 to 4 minutes, until thick.
Season with thyme, salt, and pepper. Stir in cooked chicken, frozen peas, and parsley.
Remove skillet from the heat and allow to cool slightly while you prepare the pie crust. Cut six 4¼-inch circles out of the pie crust and stick them into the bottom of the prepared ramekins. Then, cut 4-inch strips out of the remaining pie dough.
Fill ramekins with the filling until it just reaches the top. Use the dough strips to create a mummy-like weave on top of the filling, overlapping pieces every so often. Press the sides down so they stick.
Whisk the egg in a small bowl with 1 teaspoon of water. Brush egg over top of the dough. OPTIONAL: Add coarse sea salt on top of the egg for additional texture and flavor.
Place ramekins in the oven and bake for 20 to 25 minutes, or until the dough is golden brown and the filling is thick and bubbly.
Allow pot pies to cool slightly before adding candy eyes on top to look like a mummy peeking through the dough.