In a medium size saucepan, add cubed potatoes, cover with water and cook until tender; about 15 minutes.
Drain potatoes when cooked; mash and set aside.
In the meantime, in a large frying pan, add oil and butter and heat to melt on medium heat.
In a bowl, combine ground beef, ground lamb, onions, garlic, beaten eggs, Worcestershire sauce and mix well to incorporate.
Add mixture to frying pan and cook until beef is browned, about 10 minutes, stirring occasionally.
Now add carrots, petite peas, cauliflower florets, chopped zucchini, mushrooms, tomato paste, chili powder, seasoning salt, garlic salt, spaghetti seasoning and dry mustard; mix well and cook for about 5 minutes stirring often.
Prepare a 7" x 10" or a 8" x 8" casserole dish by rubbing interior with butter or spraying with a vegetable oil.
Preheat oven to 325°F.
Transfer beef mixture to prepared casserole dish, level and pat down slightly with a spatula.
Spread mashed potatoes evenly over the top.
Bake in preheated 325°F oven for 30 minutes or until potatoes are lightly browned.
Now sprinkle the grated cheese over the potatoes and return to oven and bake until cheese melts, about 5 minutes.
Remove from oven and let sit for about 5 minutes to set before cutting into 2 1/2 by 2 1/2 inch squares.
Serve with a dollop of ketchup if desired.
Refrigerate any unused portions.
Reheat 2 squares at a time in a microwave proof dish, on HIGH (full power) for 3 minutes or until nice and hot.