Prepare your mise en place: Peel, halve and thinly slice shallot. Peel and thinly slice green onion. Remove and discard shiitake stems; thinly slice caps. Peel celery root and cut into 1/4-inch dice. Peel and dice carrot. Peel potatoes and cut into 1/4-inch dice.
In a small bowl, combine 1/4 cup cold water, cornstarch, vinegar, Bragg Liquid Aminos and sriracha; stir well. Please note: The full packet of sriracha will make for a spicy ‘hot’ soup. If you like less heat, you can start with less than the full packet, and add to your taste before serving.
Place a large saucepot over medium heat; add oil. When oil is hot add shallot, and cook 2-3 minutes, stirring intermittently. Add celery root, carrot, potato and mushrooms and cook, stirring intermittently, 2-3 minutes more.
Add remaining 4 cups water and bouillon cube. Increase heat to high.
When the soup begins to boil, stir in sriracha-cornstarch mixture. Stir continuously; when the soup returns to a boil, reduce heat to low and cook, stirring occasionally, 6-7 minutes.
Increase heat to medium-high. Crack egg into a bowl, and beat to combine yolk and white. Once soup reaches a light boil (it should be bubbling hot) use a spoon to swirl it rapidly with one hand while slowly pouring beaten egg (near the edge of the pot, where soup is moving fastest) in a thin stream with the other hand. Reduce heat to low and simmer 2-3 minutes.
Remove soup from heat and let it cool for a few minutes. Taste and adjust seasoning as desired with soy sauce, salt and pepper. Divide soup into 2 bowls and top each with green onions. Enjoy!