In a large 8-quart stock pot, heat 2 tablespoons of sesame oil over medium-high heat until it shimmers, about 1-2 minutes.

Add 1 chopped sweet onion, 3 minced garlic cloves, 2 sliced carrots, and 1 tablespoon of freshly grated ginger. Sauté for about 4-5 minutes until the vegetables are lightly caramelized and aromatic.

Pour in 4 cups of chicken broth, 2 cups of beef broth, and 2 cups of water. Stir to combine and bring to a rolling boil over high heat, which should take about 5-7 minutes. Once boiling, reduce heat to low and simmer gently for at least 1 hour.

Use a skimmer or slotted spoon to gently remove the cooked vegetables. Taste the soup and add salt as needed.

Ladle the hot broth into bowls and top each serving with sliced button mushrooms and chopped scallions.