Make the chile-paste marinade: Place the chiles in three separate medium pots and cover with water (each type of chile gets its own pot). Simmer the chiles gently over medium heat until they are softened and almost completely rehydrated, about 1 hour. Remove from the heat to cool slightly; place the chiles and cooking water in a blender and purée (you may have to do this in batches).
Cool completely before using. Reserve 1 tablespoon of the paste to flavor the crema.
When the marinade is completely cool, combine it with the chicken legs, 1 cup of the lime juice, half of the diced onion, the garlic and 3 tablespoons of the salt in large, resealable Ziploc bags. Refrigerate for 3, and up to 5, days depending on how much chile flavor you want to absorb into the meat.
Braise the chicken: Preheat oven to 300°.
Place the marinated chicken legs thigh side down, legs sticking up, in a couple of large, heavy Dutch ovens or casseroles (you will most likely need more than one to make this full recipe), shingling the pieces to pack them tightly together. Add the chile purée and beer to the pan(s); the meat will not be completely covered. Don’t fret: The layer of skin on the chicken will protect the meat, and the chicken releases a lot of liquid as it cooks. Cover the pan(s) with a large piece of parchment paper and wrap tightly with foil. Braise the chicken until the joints break easily and the bones slide out of the meat without any resistance, about 1 to 1½ hours. The meat and marinade will darken as they cook, and you will start to notice a layer of rendered fat on the surface.
Carefully remove the chicken from the braising liquid to cool. Meanwhile, transfer the braising liquid to a large, heavy-bottomed pot; simmer gently until it is reduced by half, skimming any fat that rises to the surface. Remove the skin and bones from the chicken legs as soon as they are cool enough to handle; shred the meat (you will have about 6 pounds of cooked chicken).
Carefully mix the chicken with the reduced braising liquid to warm.
Assemble the tacos: Using 2 corn tortillas per taco, place a generous mound of the braised chicken in the center of the tortillas and top with a dollop of crema, fresh cilantro and chopped onion.
To make the crema: Combine 2 cups heavy cream with 1½ cups buttermilk in a ½-gallon Mason jar with a tight-fitting lid.
Let the jar sit on your counter for 1 to 3 days, until the mixture starts to thicken. When a thick layer has formed on top, mix well. Place the jar in the refrigerator to chill.
Mix the finished, chilled crema with the reserved chile paste, 2 teaspoons of the fresh lime juice and 1 tablespoon of the salt.