Before you start, whisk up cornstarch and ¼ water in a small bowl, set aside.
Heat oil in a large skillet over medium-high heat. Cook mushrooms until they start to brown and most of the moisture from the mushrooms has evaporated, about 7-10 minutes. Remove from heat, season with salt and pepper and place mushrooms on a plate.
Place the hot pan back on the element and turn heat to medium. Add butter, shallot and garlic, stirring constantly for one minute.
Add remaining 1 ¾ water, garlic powder and bouillon concentrate to pan and whisk to combine. Let come to a boil on medium, then add cornstarch slurry to pan and whisk.
Let gently bubble until mixture reaches desired thickness, about 5 minutes. Turning heat to medium low if boil gets too furious. Add in mushrooms to reheat and then you're ready to serve, if a brown gravy with lots of sliced mushrooms is what you're after!
Looking for a creamy gravy? Turn off heat. Add half to all of the sliced mushrooms into the gravy (depending on if you want some sliced mushrooms left in the gravy) and then blend with an immersion blender and serve.
Serve warm or refrigerate for up to three to four days. You could also freeze for up to one month. Reheat on medium low heat, add a tablespoon or two of water, and stir occasionally until warmed through.