Grandma's Cheesy Green Beans

This is my Grandma Evan's recipe for cheesy green beans. She adds mushrooms to the mix but I don't like mushrooms so I don't add them :)

2 T butter
2 T corn starch
1 1/2 cups milk
3 slices of american cheese
1 can green beans, drained
Salt
Pepper

Melt butter in pan. Add corn starch and stir until mixed well. Slow add milk in to the pan, mixing as you go. Let it mixture warm but not boil. Add cheese slices. Let cheese melt and then add green beans. Cook 5-7 minutes.