Place cubed potatoes, broccoli,and onions in a large pot and add enough water to cover.
Boil till potatoes can be pierced with a fork but dont fall apart.
Pour off water and add milk. If milk doesn't cover add a little extra. If you don't want to waste the water and risk losing the onions, you can add enough dry milk flakes to make milk and then proceed to next step.
Add cheese to milk and heat slowly over low heat stirring occasionally till cheese is all melted in. Add seasoning early in this process. Taste. Serve.