Step 1The night before, place the ham hock in a large pot and cover with lukewarm water. Cover and refrigerate overnight. Place the favas and a dash of salt in another pot and cover them with cold water. Let them stand overnight.
Step 2The next day, drain the favas, place them in a Dutch oven or large pot and add the ham hock, bilbao, garlic, bay leaf, pancetta, onion and olive oil. Cover with cold water and bring to a boil over medium heat. Reduce the heat to a simmer and partially cover the pot. As the favas cook, check them occasionally to make sure they're always covered with water; you can add 1/4 cup of cold water 2 or 3 times to make sure they don't break out of their skins. Stir occasionally.
Step 3Meanwhile, toast the saffron in a dry skillet over low heat, 2 to 3 minutes. Crumble and set aside.
Step 4After the favas have cooked 45 minutes, add the saffron and morcilla and continue simmering until the favas are tender, another 45 to 60 minutes. Add salt to taste.
Step 5Remove 1 cup of beans from the pot and puree them in a blender or food processor; return them to the pot. This will help thicken the broth. Remove the ham hock and cut the meat off the bone. Cut the meat into chunks and return it to the soup; discard the bone and fat. Continue cooking the soup for a few more minutes, stirring once or twice. Let rest 10 to 15 minutes before serving.