In a large pot, heat the coconut oil over medium heat.
Add the onion and cook, stirring, until softened, about 2 minutes.
Add the shiitake mushrooms and cook, stirring, for about 3 minutes.
Add the garlic and ginger and cook, stirring, until fragrant, about 30 seconds.
Add the beef bone broth, coconut aminos, fish sauce, and kosher salt. Bring to a boil; reduce the heat to medium-low and simmer, uncovered, for 5 minutes.
Meanwhile, use a spiralizer (or julienne peeler) to cut the zucchini lengthwise into long, thin strands.
Add the zucchini noodles to the simmering soup and cook until just tender, about 1 and a half minutes.
Add the sliced beef sirloin steak and simmer until just cooked, 30 to 60 seconds.
Ladle the soup into bowls and serve with the toppings of your choice.
