In researching the origins, I found that this has very little to do with Manhattan and originated in Indiana in the '40's and is more and Indiana dish. Regardless, it is still good.
I will do the breakdowns per serving and you can multiply it as needed
I started used instant mashed potatoes and made them according to the the instructions on the box.
I used sliced roast beef from the deli but leftover potroast works well also. Put on a microwave safe plate and nuke on high for approx. 2 mins.
For the bread, use 1 to 2 slices per person as the base. To toast or not to toast is a personal choice, I usually toast very lightly in the oven for a minute.
Open gravy and warm on medium heat on the stove.
Put 1 to 2 slices of bread or toast on a plate.
Put a large scoop of mashed potatoes on top.
Use a fork to put slices of roast beef on top of the bread or toast.
Ladle gravy over the top of the roast beef, then serve.