Sort through the beans to make sure there are no little stones and then put them in a large bowl or pot. Add enough cold water to cover the beans by 2 inches and let soak overnight. Drain the beans well, tilting the colander instead of shaking it to gently extract the water and protect the beans' skins.

Position a rack in the center of the oven and heat the oven to 300 degrees F.

Heat the olive oil in a 5- to 6-quart Dutch oven over medium heat. Add the chorizo and cook, stirring occasionally, until nicely browned, 6 to 8 minutes. Transfer with a slotted spoon to a small bowl. Depending on the amount of fat left in the pot, pour off and discard all but 3 tablespoons or add enough olive oil to make 3 tablespoons. Add the onion and garlic and cook, stirring occasionally, until softened, 5 to 7 minutes.

Add the broth, bay leaves, cumin, cinnamon, and 1/2 teaspoon pepper. Stir to combine. Add the beans and bring just to a simmer over high heat. Cover and bake until the beans are easy to bite into but still a little mealy in texture, 45 to 60 minutes.

Stir in the reserved chorizo, 1 cup sherry, and the tomatoes, brown sugar, and thyme. Bake, uncovered, until the beans are fully tender, 30 to 60 minutes more. Cool to room temperature, remove the bay leaves and thyme stems, cover, and refrigerate overnight.

To finish, bring to a simmer, uncovered, over medium heat. Reduce the heat to low and continue to simmer until the sauce is reduced to the consistency of thin gravy, stirring occasionally so the bottom doesn't burn, 40 to 60 minutes. Stir in the remaining 2 tablespoons sherry (if using) and season the beans to taste with salt and pepper.

Put them in a 4-quart saucepan. Add enough cold water to cover by 2 inches and bring just to a boil. Remove from the heat and soak for 2 hours. Drain the beans and continue with the recipe.