In a saucepan or Dutch oven, cook the onion in the oil over medium heat until tender.
Add the beans, fresh water, and diced fennel or avocado leaf. Increase the heat, bring to a boil, and keep at a boil for two minutes.
Turn down the heat, cover, and cook at a low simmer until the beans are tender (at this point, you could also transfer the mixture to a slow cooker or move the pot to a solar oven to complete the cooking).
When the beans are about one-half to two-thirds cooked, add the salt, starting with 2 tsp.
In a small pan, cook the chorizo or Soyrizo, breaking it up into small pieces, until it is lightly browned. Set aside.
Near the end of the bean cooking process, add the chorizo to the beans, stir, and adjust the salt to taste. Remove the avocado leaf before serving.