Place an oven rack on the lowest position and heat the oven to 425 F degrees
Melt butter in a medium-sized pot over medium heat.
Add onions, carrots, celery, salt, and pepper. Saute' until vegetables are slightly soft for 5 minutes.
Add the garlic, mushrooms, sage, thyme, and Italian parsley and saute'.
Add flour and cook, stirring continually until fully incorporated.
Slowly stir in the cream and chicken broth and bring to a low boil on medium-high heat.
Stir in the shredded Fontina cheese.
Stir in the potatoes and peas.
Reduce heat to medium and simmer until the sauce is thick and the potatoes are tender, about 10 minutes.
Now taste the filling's sauce ... add 1 tsp. of salt if you prefer (we do) and add the chicken bouillon.
Take the pot off the heat and stir in the chicken. Add a few more tablespoons of cream if the sauce is too thick.
Place one layer of pie dough on the bottom of the largest pie pan you have, preferably a 9-½" pie pan -- if you don't have a larger pie pan, then this will make one regular pie size plus one small pie or small casserole. It is a LOT!
Roll out the second, top layer of pie dough and then very loosely place it around the rolling pin. Carefully place it over the filling and gently 'unroll' the dough onto the filling.
Trim overhanging extra dough to about ½" from the edge of the pie dish.
Pinch the bottom layer of pie dough with the top layer of pie dough with your fingers, pressing the two layers of pie dough firmly together.
Cut four 2" long slits on top of the pie dough.
Brush the top of the pie and edges with beaten egg.
Place the pie on a baking sheet or pizza pan on a lower oven rack.
At 425°F (218°C), bake until the top is light golden color, for 20 minutes.
With aluminum foil cover the edges of the pie crust or use a pie crust shield to prevent the edges from becoming too brown.
Reduce oven temperature to 375°F degrees, rotate the pot pie, and bake for another 10 minutes until the top crust is a deep golden brown. Insert an internal thermometer; the center of the pot pie should be 165 F degrees.
Allow the pie to set and cool on a wire rack for at least 45 minutes.
Garnish with minced Italian parsley