Preheat oven to 450 for baking crusts, then turn down to 350 for baking pies.
1. Roll out pie crusts and measure by tracing the outlines of the foil tins, approximately ½ inch past the rim onto the dough, as shown. If using a muffin tin, measure the diameter of the muffin cup and add ½ inch. Place into the greased tins and overlap the dough to make fit. Prick the dough with a fork over the bottom and sides, and bake for approximately 8 minutes or until brown. When finished, let cool.
2. Meanwhile, in a skillet, sauté the onion and celery till translucent and add the beef. Cook thoroughly and add the seasonings. Sprinkle ½ tbsp. of flour over the beef and add the frozen vegetables. Stir together to dissolve the flour and then add 1 C of the beef stock. Continue to stir until stock thickens and forms a gravy. Take off the heat and set aside.
3. In another pot, boil the potatoes until soft, drain and mash with cream and 1 Tbsp. of the butter. Set aside, covered.
4. Take the crusts and spoon in the hamburger mixture until it reaches the top of the crust. Pack down slightly. Then, spoon the mashed potatoes into a mound on top of the meat. Smooth out if you wish or leave a rustic look. Repeat with the additional 5 dishes.
5. Bake at 350-degrees F until the heated through.
6. In a medium saucepan, combine the butter and flour over medium high heat and whisk together. Cook for 1-2 minutes to remove the raw flour taste.
7. Whisk in the remaining beef stock. Cook for an additional 2 minutes.
8. Top each pie with a spoonful or two of the gravy, as desired.