Preheat oven to 375 degrees F.

In a medium saucepan, bring stock or broth to a simmer. Add chicken and poach until just cooked through. With a spider or slotted spoon, remove chicken and set aside. Set stock aside for later use.

In a 10-quart saucepot, melt butter over medium heat and then add onions, carrots, and celery and saute for a couple of minutes. Add mushrooms and continue to cook until onions are about transparent, being careful not to brown. Add flour and gently mix well. Cook this mixture over low heat for 6 to 8 minutes, stirring occasionally and gently so as not to break up the vegetables. Scrape the bottom of the pan often with a wooden spoon so that the roux doesn't burn. Increase the heat and add the chicken stock in 3 additions, whisking well after each so that no lumps appear and returning to a brief simmer each time.

Add bay leaf, salt, and pepper, and cook over low heat for about 10 more minutes. Add poached chicken and warm heavy cream, and stir well. Spoon equal portions into 6 individual (16-ounce) ovenproof casserole dishes or bowls. Top with your favorite pie dough or puff pastry.

In a small bowl, whisk together 2 egg whites with a little water. Brush dough with egg wash. Place casseroles on a sheet pan and bake until golden brown on top. Remove from the oven and let cool for 5 to 10 minutes before serving.