If you’ve ever thought that there must be a way to make really flavorful onion soup without resorting to onion soup mix, miso is the secret ingredient you’ve been seeking. Miso comes in a variety of flavors, and you can play around to see which one suits you best. Sweet white or mellow miso has a sweet and salty taste, but may be harder to find in local health food stores. Brown rice miso has a stronger flavor, but may be more readily available. Miso can be stored in the refrigerator for a long time, so you never need to worry about having it on hand for a recipe.
Heat oil in a 6-quart pot over a medium flame. Add the onion rings and sauté. When the onions begin to sizzle, add a pinch of salt and continue sautéing until golden brown, approximately 30 minutes.
Add the mushrooms and water, then cover and bring to a boil. Add salt to taste and sprinkle lightly with mirin. Reduce heat to low and allow to simmer for 45 minutes to 1 hour.
Remove a half of a cup of soup and place in a small bowl. Add the miso to the bowl and stir until fully integrated.
Then return the miso mixture to the pot and simmer uncovered for 5 minutes. (Do not allow the soup to boil once the miso has been added.) If desired, add a handful of shredded cheese as it is simmering, just before serving.
This soup can be reheated, but only on a low flame.