Preheat the oven to 350 F, or 175 C. In a large saucepan, heat the beef stock until boiling, and add in the chopped carrots, celery and potatoes. Allow to boil until softened, about 8-12 minutes, depending on the dice. **The smaller you chop these, the quicker they will soften in the boiling water. Once softened, remove the carrots, celery and potatoes, but leave the excess beef stock in the pot.
Heat the oil in a medium-sized frying pan and brown the beef along with the onions. Remove from heat once browned and set to one side. Pour the softened vegetables in with the beef and add the frozen peas, salt, pepper and thyme.
Make a quick gravy by mixing together the corn starch with the water until dissolved. Pour this into the beef stock and continue to stir over a medium heat until it thickens to a gravy.
Assemble the casserole by placing one of the pie crusts into the bottom of a 1 1/2 quart round casserole dish. Spoon the beef and vegetables over the crust and pour the gravy over the top. Place the second pie crust over the top and use a sharp knife to score the crust three times (cut three parallel lines in the top of the crust). Place into the oven and bake for 25 minutes, or until the crust is golden and cooked through.