Place the turkey leg in a large heavy pot. Add the cold water, epazote sprigs and Mexican oregano. If using fresh turkey add a generous pinch of salt now. If using smoked turkey add no salt at this stage. Bring to a boil over high heat. Skim the grey foam that rises to the surface continuously for the first 10 to 15 minutes. This step matters and should not be skipped. Reduce to a gentle simmer, cover and cook for 1 hour to 1 hour 15 minutes until the turkey is completely tender and the meat pulls easily from the bone.

Remove the turkey from the broth. Set aside to cool slightly. Strain the broth through a fine mesh strainer and reserve. You need approximately 3 cups. Shred the cooled turkey into large rough pieces removing all bones and skin. Set aside.

Place the pumpkin seeds in a dry heavy pan over medium heat. Toast, stirring constantly, until they begin to pop and turn lightly golden, about 2 to 3 minutes. Remove immediately. Do not let them burn.

Using the same dry pan over medium heat, press each dried chile flat against the surface one at a time for 20 to 30 seconds per side until fragrant and slightly darkened. Watch them carefully. A burnt chile will make the entire sauce bitter and cannot be corrected. Place all toasted chiles in a bowl and cover completely with boiling water. Leave to soak for 20 minutes until completely soft and pliable.

Drain the soaked chiles and place in a blender with the roughly chopped tomatoes, toasted pumpkin seeds, the epazote sprig if using, and 1 cup of the reserved turkey broth. Blend on high for 1 to 2 minutes until completely smooth. The sauce should be a deep reddish brown, thick and intensely fragrant. For a smoother, more refined result pass through a fine mesh strainer pressing all the solids through with the back of a spoon. This is optional but produces a sauce closer to the standard of preparation that would have been served at Montezuma's table.

Heat the lard in the heavy pot over medium high heat until shimmering and very hot. Pour the blended chile sauce directly into the hot lard all at once. It will splatter aggressively for a few seconds. Stand back slightly. This step of frying the sauce in fat is essential. Stir continuously for 3 to 4 minutes until the sauce darkens slightly, thickens and the raw flavour cooks off. The aroma at this stage should be deeply complex, smoky from the chiles and earthy from the tomatoes and pumpkin seeds.

Add 2 cups of the remaining reserved turkey broth to the fried sauce and stir to combine. Return the shredded turkey to the pot. Bring to a gentle simmer and cook uncovered for 20 minutes, stirring occasionally, until the sauce has thickened to a coating consistency and completely coats the turkey. Taste and adjust salt carefully, especially if using smoked turkey which may need very little or none at all.

Warm the corn tortillas directly over a gas flame or in a dry pan until pliable and lightly charred in spots. Serve the turkey and chile sauce directly from the pot into bowls or spooned onto the warm tortillas at the table.