Start by toasting your spices. (Toasting the spices will give you a richer flavor, which I think is important in this recipe where we're skipping out on bones/fat/meat.) Heat a small saucepan over medium-low heat. Put the cinnamon stick, star anise, whole cloves, cardamom seeds, and fennel seeds in the sauce pan and heat them until lightly toasted, about 3-5 minutes. Shake the pan often to prevent burning. The spices should be aromatic; set aside.
Bring 4 cups of vegetable broth to a simmer in a medium-large saucepan over moderate heat. Add the tamari or soy sauce, and if you're pescetarian, the fish sauce as directed. Add the thinly sliced onions.
Place the spices in a cheesecloth or steeping ball. (If using a steeping ball, put everything in it except the cinnamon stick since it won't fit, and simply place the cinnamon stick into the broth. You can easily fish it out later.) Place the vessel into the broth to steep for 15 minutes while the broth simmers.
While the broth simmers, prepare the rice noodles as per the package's instructions.
Once the rice noodles have been cooked, drained, and rinsed, transfer them to serving bowls.
Remove the spices from the broth and give the broth a good stir. Carefully pour the broth over the noodles and garnish the soup with your choice of toppings + sauces. Give your bowl a refresh spritz of lime before eating.