Preheat to 400°F and grease an 8 x 10 inch baking dish.
Sauté onions, carrots, and parsnips in a small amount of vegetable oil over medium heat until tender.
Add the ground meat and sauté until no pink remains. Next add the thyme, Worcestershire Sauce, and season with salt and freshly ground pepper.
Add the beef stock and cook, uncovered, over low heat for about 10minutes, adding more stock as necessary to keep moist.
Add the frozen peas, and spread the mixture evenly in the baking dish.
Add the mashed potatoes on top of the mixture. For a special treat, fill a large pastry bag with a fluted end and pipe the potatoes on top of the meat, but be sure your potatoes are well mashed or the lumps will clog your piping tube.
Place the piping bag in a tall flower vase to help you fill it.
Bake for 30 minutes or until the potatoes are lightly browned.