Heat a large pot over medium-high heat and add 2 tablespoons of sesame oil. Once shimmering, add 1 diced sweet onion, 3 minced garlic cloves, 1 cup of diced carrots, and 1 tablespoon of freshly grated ginger. Sauté for 5-7 minutes until onions are translucent and garlic is fragrant.

Pour in 4 cups of chicken broth, 2 cups of beef broth, and 4 cups of water into the pot, stirring to combine. Bring to a boil, then reduce heat to low and simmer for at least 1 hour.

Use a slotted spoon to skim off any foam or impurities. Carefully remove the vegetables using a fine mesh strainer.

Taste and season the broth with salt and soy sauce if desired. Ladle the hot broth into bowls and garnish with sliced button mushrooms and chopped scallions.