Put the onion and epazote in a pot and add the broth, plus another quart of water. Add the lentils and bring to a simmer. Simmer until the lentils are tender, but not mushy. You want them cooked, but still a little al dente. Add salt to taste and turn off the heat.
Meanwhile, in another pot, cook the chorizo -- it should be fatty enough so you don't need to add more fat or oil -- until well rendered, then add the onion, carrot and celery. Cook these until soft, then add the garlic and cook for another couple minutes.
Stir in the cumin, green chiles and tomatoes. If the lentils are done, add them, plus as much of the lentil cooking liquid as you want. If the lentils aren't done, just turn the heat off the soup base until they are.
Once everything's all together, simmer another few minutes, adding salt to taste. Serve with chopped cilantro and as many chile pequins as you can stand.