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1
In a large bowl, soak the kidney and pinto beans in cold water overnight.
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2
Soak the black beans overnight in another bowl.
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3
Drain the two bowls of beans separately and transfer each batch to a large saucepan.
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4
Add 1 ham hock or piece of bacon to each saucepan.
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5
Add 2 quarts of the water and 4 cups of the chicken stock to the pinto and kidney beans.
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6
Add the remaining 1 quart of water and 2 cups of chicken stock to the black beans.
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7
Bring the beans in the saucepans to a boil, then cover partially and cook over low heat until they are almost tender, about 1 hour for the kidney and pinto beans and 35 minutes for the black beans.
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8
Drain the beans; reserve the kidney and pinto bean cooking liquid.
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9
Discard the black bean cooking liquid.
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10
Remove the meat from the ham hocks and shred; reserve the ham hock fat and 1 bone.
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11
If using bacon, trim the fat and cut the meat into pieces.
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12
Reserve the bacon fat.
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13
Heat a medium cast-iron skillet.
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14
Add the ancho chiles and toast, pressing down with a spatula until they are fragrant and blistered, about 30 seconds.
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15
Transfer to a heatproof bowl, cover with boiling water and soak until softened, about 15 minutes.
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16
Drain and finely chop.
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17
In a large, enameled cast-iron casserole, cook the ham hock fat or bacon fat in the vegetable oil over moderately high heat until the fat is crisp, about 5 minutes.
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18
Discard the fat.
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19
Add the onions, jalapenos, garlic and ancho chiles and cook over moderate heat, stirring occasionally, until softened, about 8 minutes.
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20
Add the reserved ham or bacon meat and cook, stirring until golden, about 3 minutes.
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21
Add the barbecue sauce, tomato paste, the reserved ham hock bone and all of the drained beans.
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22
Stir in 4 cups of the reserved bean cooking liquid and season with salt and pepper.
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23
Cover and cook over moderately low heat, until the beans are tender and the gravy is thick, about 1 1/2 hours longer; add more of the bean cooking liquid if the beans seem dry.
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24
Discard the ham hock bone and transfer the beans to a large shallow baking dish or individual crocks.
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25
Preheat the oven to 425.
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26
In a medium bowl, toss the cracker crumbs with the cheese.
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27
Sprinkle the mixture over the beans.
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28
Bake the gratin for about 25 minutes, or until bubbling and golden.
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29
Serve with lime wedges.
