Bring to a boil, then cook on medium heat for 10-15 minutes (or when potatoes are tender).
Add chicken bouillon cubes, salt and pepper. Mix.
In another pan, melt butter on medium heat. Whisk in flour and stir constantly for about 1 minute. Slowly add milk and continue whisking until sauce thickens (about 3-4 minutes).
Add the milk mixture to your pot and cook soup until heated through.
Add cheese right before serving. Top with scallions and extra cheese if desired. Serve immediately.