Place a large 8-quart stock pot over medium-high heat. Add the oil and place the onion, garlic, carrots, and ginger in the pot. Sear the veggies on all sides to caramelize, making sure not to burn the garlic.
Pour in the chicken broth, beef broth, and water. Bring to a boil. Lower the heat to a low boil and simmer for at least one hour.
Use a skimmer to remove the vegetables from the broth. Taste, then salt as needed.
To serve: ladle hot broth into bowls and sprinkle chopped scallion and sliced mushrooms over the top.
In restaurants, this soup is made several days in advance to allow the flavors to deepen. It's not only a great leftover; it actually tastes better on the second day. If you have time, make it ahead, then rewarm the soup before adding the scallions and mushrooms.