Cover the beans with plenty of cold water and soak for 12 hours. Drain and rinse.
Place beans in a Dutch oven with 8 cups fresh water, onion, garlic, bay leaves, and saffron. Bring slowly to a bare simmer.
Slow-Simmer:
Add chorizo, morcilla, and panceta. Simmer uncovered or partially covered for 2 to 2 1/2 hours, adding small amounts of cold water when needed to keep the beans submerged.
Skim excess fat, shake the pot gently instead of stirring, and cook until the beans are creamy-tender and the broth lightly thickened.
Rest and Serve:
Remove onion, garlic, and bay. Slice the meats into serving pieces, return them to the pot, season with salt, and rest for 20 minutes before serving.
Notes
Salt near the end because cured meats vary greatly in salinity.
Keep the stew at a quiet simmer; hard boiling can split the bean skins.