In a medium bowl whisk together the soy milk, flour, mushroom boullian, and pepper. Whisk well to make sure there are no lumps. Set aside.
Melt the vegan butter in a sauce pan over medium-high heat. Add the onion and garlic and sauté until the onions turn translucent and begin to brown, about 5 minutes. Add the mushrooms and continue to cook until the mushrooms have browned, reduced in size and begin to release juices, another 5 minutes.
Soy milk mixture in with the mushrooms and continue to cook, stirring often, until the condensed soup has thickened, about 3 more minutes. The soup will thicken even more once cooled. Store in a sealable jar in the fridge for 3 - 5 days or it can be frozen for later. This recipe is equivalent to one can of soup. You can use it in casseroles or where other recipes call for condensed soup, or you can use it to prepare soup .