Add your olive oil or vegan butter to a pan over medium heat. Allow it to coat the pan before adding your onions, and reducing the heat to low-medium. Add the minced garlic and sea salt, and sauté for up to 20 minutes, stirring often, until the onions are caramelized.
In the meantime, combine your “au jus” ingredients in a measuring cup or jar. At this time, you can also make your cheese sauce by combining the listed ingredients in a blender until smooth. Taste and adjust the salt content as needed.
In a separate pan from your onions, begin sautéing your mushroom strips until a lot of the water has evaporated out of them, for approximately 5 minutes. Add 1/2 a cup of the Au Jus mixture and sauté until the mushrooms are softened and combined with the sauce, approximately 5 additional minutes. Remove the mushrooms and heat the remaining au jus sauce, and transfer to a container for dipping.
Assemble your sandwich the Portobello mushrooms first, then caramelized onions, followed by a drizzle of the “cheese” sauce. Fry your sandwich in your empty onion pan as desired. Dip in the remaining sauce and enjoy!