In a medium-sized skillet, melt butter over medium heat.
Add mushrooms and soy sauce and sauté for about 5 minutes or until mushrooms have cooked down and very tender.
Sprinkle flour over the top of the mushrooms and stir until a thick paste forms. Cook for another minute or two until flour is well incorporated.
Slowly add milk while whisking constantly. Whisk until smooth and creamy. Add beef bouillon and stir until well incorporated.
Add salt and pepper to taste.
You want this to be slightly thicker than gravy (it is a condensed soup after all) so keep that in mind if it seems a little thick. It will also thicken up upon cooling.
Allow to cool and place in an airtight container. Refrigerate or freeze until ready to use.
Notes: This recipe makes approximately 3 cups of condensed soup. A store bought can of cream of mushroom soup (10.75 ounces) is 1 1/4 cups. Just something to keep in mind when using in a recipe :)If you are wanting to make this into a soup-- just use an equal amount of liquid (such as milk or broth) as condensed soup (Example: if you use two cups of condensed soup, add two cups of liquid)1 serving = 1/2 cup prepared