I started making my potato cheese soup this way when I was left with a big pan of mashed potatoes from the night before. I just couldn't bare to toss them, so I made them into my favorite soup. It is so easy. Now I make an enormous amount knowing I will make soup with the leftovers.
First, start off with a good amount of left over mashed potatoes. I have a family of six so you may need less.
You then need a can of chicken broth or some made from bullion and water. Milk or half and half and cheddar cheese. I like sharp cheddar.
Mix the can of chicken broth into the potatoes stirring until hot. Then add the milk or half and half about 2 cups. Season with salt and pepper but the cheese is salty so you don't need much. I used both half and half and milk so it wasn't too fattening. Heat until bubbly. If you like it thick, like I do, add some flour and water and reheat to bubbling. Just before serving add the cheese until melted.
I forgot to snap a pic all nicely served in a bowl. You can see from the pan that my family loves it.