Fill a large pot of water with just enough water to cover the potatoes and bring it to a boil.
Reduce the heat to a simmer and continue to cook the potatoes for about 15-20 minutes until soft. When poked with a fork, the potato should mostly fall apart.
Remove from heat and drain most of the water, from the pot, leaving just a half-inch of water at the bottom of the pot. (This doesn't have to be exact so don't stress!)
Add the milk garlic salt and pepper, then gently mash the potatoes with the back of a fork until the potatoes start to break down. I like my soup chunky, but if you like it smoother, you could use a potato masher or an immersion blender.
Continue to cook on low heat for 5 minutes.
Divide into bowls and top each bowl with a little cheese, green onions, and additional salt and pepper to taste