Using a fork, whisk the flour, and the beef flavored Better Than Bouillon with 1/2 a cup of the milk until smooth. Set aside.
In a saucepan over medium-high heat, melt the butter. As soon as the butter is fully melted and hot, add the mushrooms to the saucepan, toss in the butter, season with salt and pepper to taste, then spread the mushrooms out in a single layer.
Sauté the mushrooms, undisturbed, until the bottoms brown, about 4 to 5 minutes. Flip the mushrooms and cook for another 2 minutes.
Reduce the heat to medium and add the remaining cup of milk to the saucepan, continually whisking with a fork as the milk heats up.
Once the milk in the saucepan begins to get frothy and boil, slowly add the milk and flour mixture to the saucepan, continually whisking as you add it.
Turn off the heat just before the soup reaches your desired consistency. Continue whisking for another minute or two as the soup cools.
Use as a substitute for any recipe that calls for canned cream of mushroom soup.