In a skillet heat bacon and cook until bits have rendered. Strain and reserve the bits for later. In remaining fat in skillet (add some oil if not enough) add onions and saute for 5 minutes or until tender. Add garlic and mushrooms and continue to saute a couple of minutes or until cooked through. Remove the skillet from the heat and strain out any excess fat and liquid by pouring contents through a sieve; add bacon bits to onions and mushrooms in a mixing bowl. Stir in leftover pot roast and peas and set aside. In a small saucepan, heat 3 tablespoons of butter and add the flour. Cook for a minutes to make a roux. Add broth and whisk vigorously . Bring to a simmer and cook for a few minutes or until thickened. Add dill weed and pickle and adjust the seasoning.
Preheat oven to 475 degrees. Pour sauce over the beef, peas, etc. and season to taste with salt and pepper. Pour the mixture in a quartsize, 2 inch deep round baking dish. Roll out pastry and set over the pie. Tuck excess dough under the rim and crimp the edges decoratively if you wish. Cut a couple of slits in the crust to allow steam to escape. Set pie on a baking sheet and set in middle of oven. Bake at high heat for 15 minutes. Lower heat to 375 degrees and bake for 15 minutes more or until crust is golden and the filling is hot.