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1
Put the cumin and coriander in a small pan and set over medium heat. Toast the spices for a minute or two, shaking the pan occasionally, until they're fragrant. (Be careful not to scorch them.) Grind the spices finally using a spice grinder or a mortar and pestle. Transfer to a small bowl, stir in both chile powders and the oregano and set aside.
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2
Heat a tablespoon of oil in a large, heavy pot or Dutch oven over medium-high heat. Season the pork well with salt and add about half of the meat to the pot. Brown it well on all sides, about 4 minutes total, and transfer it to a bowl using a slotted spoon. Add more oil and brown the second batch of meat, adding it to the bowl once it's done.
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3
Lower the heat to medium and add another tablespoon of oil to the pot. Add the onion, poblano, serrano and a generous pinch of salt. Cook, stirring frequently, until softened, about 5 minutes. Add the garlic and cook for a minute or so, until fragrant. Stir in the spice mixture and cook for another minute.
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4
Return the meat to the pot, with any juices that have accumulated, and add the tomatoes and chicken stock. (The meat should be covered in liquid - if it's not, add a little water.) Squeeze in the juice of half a lime and add 2 teaspoons of salt. Turn the heat up to high and bring the liquid to a boil. Turn down the heat, cover the pot and simmer gently for an hour. Uncover the pot and continue to simmer until the pork is very tender and the sauce is reduced and thick, 30 to 60 minutes longer. Taste for seasoning, adding salt if needed, and squeeze in the juice of another half a lime.
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5
To serve, heat the tortillas in the oven or in a dry pan. Peel and roughly chop the avocados and cut the remaining lime into thin wedges. Fill each taco with some of the pork, pickled onions, avocado, cilantro and sour cream if you like. Serve with lime wedges for squeezing.
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6
Slice the onion thinly and put it in a microwaveable container. Add the salt, sugar, beet and cilantro. Cover everything with 1 part water to 2 parts vinegar. Microwave for 1 minute, stir, and microwave for another minute. Cool, then cover and refrigerate overnight.
