Add the vegetable oil to a large soup pot. Break the vermicelli noodles into one to two inch sections and then add them to the pot. Cook the dry noodles in the oil over medium-low heat while constantly stirring for 3-5 minutes, or until the noodles have turned golden brown and are slightly blistered. You may need to brown the noodles in batches.**
Add the diced onion, minced garlic, and cumin to the pot with the noodles and continue to cook and stir for a few minutes more, or until the onions have softened.
Add a little of the juice from the can of tomatoes to the pot to stop the browning of the noodles. Use a blender or immersion blender to purée the canned tomatoes along with their remaining juices. Add the puréed tomatoes to the pot with the noodles, along with the six cups of chicken broth.
If using a jalapeño pepper, add it to the pot whole (this gives a slight jalapeño flavor without too much heat). Place a lid on the pot, turn the heat up to medium high, and allow it to come to a boil. Let the pot simmer for about 15 minutes, or until the noodles are soft. If the noodles absorb too much liquid, feel free to add more tomato sauce or broth if desired.***
Add lime juice and roughly chopped cilantro leaves to the finished soup just before serving. Feel free to slice the jalapeño and use it as a garnish when serving.
*I use Better Than Bouillon soup base to make my broth because it's less expensive than canned broth, and I can mix up any amount that I need. (1 tsp chicken base + 1 cup water = 1 cup broth). For a vegetarian soup, use vegetable broth instead. It won't change the flavor too much, and it'll still turn out delicious.**When toasting the noodles,spread them in an even layer, and stir often so the top noodles move to the bottom. They should turn golden brown and look slightly blistered, but watch closely because they can burn fast. Toast in batches if your pot is small.***I like to serve this soup right away because the noodles will keep soaking up broth and can become mushy as they sit. If the soup looks too watery, simmer it uncovered for a few extra minutes so the broth reduces and the noodles absorb more liquid.