1. Soak the pasilla and ancho chilis in hot water for 30 minutes. Stem and seed, set aside.
2. Saute the onion in a little oil. When the onions are translucent, add the garlic. Cook for a minute and then add the tomatoes and cook until the tomatoes are tender (about 5 more minutes). Transfer tomatoes, onions, and garlic to a blender. Add the chilis and blend.
3. In a large pot, bring the salsa to a simmer. Cook for a few minutes (until it starts to smell good), then add the chicken stock and bring to a boil. Lower heat to a simmer. Add the epazote and season with salt.
4. Serve topped with tortillas, queso, and sliced avocado
Serves 3 to 4