1. Preheat oven to 400.
2. Spread the bones and onion out on a large baking pan and roast for 40 minutes to one hour. (This step is optional but really bring out a lot of the flavor in the beef broth).
3. Put the bones and onion in a large stock pot with the garlic, star anise, ginger, fish sauce, and some salt and pepper. Cover everything with the water and bring to a boil. Reduce the heat to low and allow the pot to simmer for at least 6 hours, or up to 24 hours. Strain the broth and use immediately or refrigerate until ready to serve.
4. Bring the broth to a simmer and add the sliced beef. While the beef is cooking, divide the zucchini noodles among 2 to 4 serving bowls. Once the beef is pink (should take 5 to 7 minutes), spoon broth over the noodles and top the soup with cooked beef. Add cilantro, green onion, bean sprouts, basil, lime, and chili garlic oil as desired and serve.

Variation 1: Quicker Pho (What I did)

1. Use store-bought beef broth if you don’t have the time to make the broth yourself – just quickly char the onion, garlic and other spices in a dry skillet before transferring to a large pot. Pour the beef broth overtop and bring everything to a simmer. Strain the broth and proceed as directed in the original recipe.