Sauté the onions in a pot in a little bit of oil until slightly carmelized. About 10 minutes.
Add the carrot, celery, garlic, and ginger, sesame oil, and broth. Season to taste with salt and pepper.
Bring to the boil and then simmer for 30 minutes.
Strain the veggies from the broth.
Add a handful of scallions and thinly sliced mushrooms to bowls. Ladle the soup on top.
Optional: Add a splash of soy sauce and sriracha to taste.