Heat a little oil in a large pot over medium heat. Add the diced onions and cook for about 10 minutes, stirring occasionally, until they become soft and slightly caramelized.
Stir in the carrots, celery, garlic, and ginger. Pour in the sesame oil and vegetable broth. Season with salt and pepper to taste.
Bring the mixture to a boil, then reduce the heat to a simmer. Let it cook uncovered for 30 minutes to allow the flavors to develop.
Use a fine-mesh strainer or slotted spoon to remove the vegetables, leaving a clear broth. Discard the solids or save them for another use.
Place a handful of thinly sliced mushrooms and scallions into serving bowls.
Pour the hot broth over the mushrooms and scallions, allowing them to soften slightly in the heat.
Add a splash of soy sauce for extra depth or sriracha for a spicy kick.