Cook onion, beef, thyme and pepper (to taste) in 12 inch (30 cm) ovenproof skillet or gratin dish over medium-high heat, breaking up meat with wooden spoon, until beef is browned and onion is softened, about 10 minutes.
Reduce heat to medium; stir in Worcestershire sauce, tomato paste and flour. Stir in vegetables and stock; simmer until slightly thickened, about 3 minutes.
Gradually stir half of the cheese into potatoes; spread evenly over beef mixture. Top with remaining cheese; bake in 425°F (220°C) oven until lightly golden, about 25 minutes. Broil until topping is rich golden-brown, about 3 minutes.
Buttermilk Mashed Potatoes: Cover 1 1/2 lb (750 g) potatoes, peeled and cubed, with cold salted water. Bring to boil over medium-high heat; reduce heat and simmer until tender, about 25 minutes. Drain and return to pot over medium heat; heat until dry, about 1 minute. Remove from heat. Stir in 1/2 cup (125 mL) buttermilk and 1/4 cup (50 mL) butter; mash until smooth.
*Other Options: Use Lean/Extra Lean Ground Beef