In a large saucepan, melt the butter on medium-high heat. Add in the mushrooms and saute until tender, about 5 minutes.
Slowly stir in the flour until there are no clumps.
Add in the beef broth, and heat to a boil.
Add in the potatoes, reduce to a simmer and cover for 15 minutes (until potatoes are tender), stirring occasionally.
Add in the half and half, and stir until heated through (you don't want it to boil, just get hot!). Add the salt, pepper, and garlic salt to your liking and serve.
Notes
This recipe could most definitely be made into a freezer meal. I will sometimes make a double batch, then place the other half in gallon bags, once it has cooled down. I would eat it within 2 months of making.