In an 8-quart stock pot, heat 2 tablespoons of sesame oil over medium-high heat. Add 1 diced sweet onion, 2 smashed garlic cloves, and 1-inch knob of freshly grated ginger. Sauté for about 3-4 minutes until the onions are translucent.

Pour in 6 cups of chicken broth, 4 cups of beef broth, and 2 cups of water. Raise the heat and bring the mixture to a rolling boil, which should take about 5-7 minutes.

Reduce the heat to low, allowing the soup to simmer gently for at least 1 hour to meld flavors.

Carefully use a skimmer to remove all the vegetables, leaving just the clear broth. Taste and season with salt as preferred.

Ladle the broth into bowls and garnish with thinly sliced button mushrooms and chopped scallions.