Preheat oven to 400°F.
Place the potatoes in a pot, cover with about 1-2 inches of water, add salt. Bring to a boil and cook for 20 minutes or until tender. Then, drain the water and mash the potatoes with 4 tbsp of butter, salt, and pepper.
In a large skillet, melt 4 tbsp of butter, add onion and cook for 5-10 minutes or until tender. Add vegetables, simmer until ready. Then, add beef, cook until brown. Add salt, pepper, beef broth and Worcestershire sauce, cook uncovered for 10 minutes. Do not allow the beef filling to become dry.
Spread beef filling on the bottom of the 8x13 casserole dish, then evenly spread the mashed potatoes and flatten with a fork, creating peaks.
Bake in the preheated oven for 15-20 mins until lightly browned.