You should prepare the meat filling and mashed potatoes simultaneously so they are ready at the same time.
Preheat the oven to 375 and lightly grease a 3-quart casserole dish.
Brown the beef in a large skillet over medium-high heat. Drain. Add frozen vegetables to skillet and heat over medium while making the gravy.
Melt the butter in a small saucepan. Whisk the flour into butter, cook over medium heat for 2 minutes. Add stock, ketchup, Worcestershire and cook for 5 minutes to thicken. Season with salt and pepper. Pour over meat and veggies and stir to combine.
Spoon meat into casserole dish.
Peel and quarter potatoes and put into large saucepan. Cover with water and boil for 20 minutes.
Drain potatoes. Mash the potatoes with masher or electric mixer and beat until smooth. Add the milk and butter. Stir in the cheese if desired and season the potatoes to taste with salt and pepper.
Using 1/2 the mashed potatoes, spread a thin layer over the entire casserole. Spoon the remaining mashed potatoes into large ziploc bag and cut opening about 1/2" wide off one bottom corner. Squeeze conical ghosts about 3" tall onto the prepared casserole dish of shepherd's pie, pushing down the bag to make a thicker base and then tapering upward to a narrow head. Add a couple of seeds on the heads for eyes.
Bake pie and potatoes for 20-25 minutes until the sauce is bubbling serve immediately.