Simmer the turkey. Put the turkey legs in a large pot with the broth, and add enough water to cover everything by 2 inches. Bring to a simmer and cook, partially covered, until the turkey is tender, anywhere from 90 minutes to 3 hours depending on the age of the bird.
Meanwhile, make the sauce for the soup. Put the halved tomatoes, quartered onion, garlic cloves and tomatillos, cut side up, on a baking sheet and broil until partially blackened, about 15 minutes. Keep an eye on them so they don't completely incinerate.
Open the dried chiles and remove the seeds and stems. Pour boiling water over them in a small bowl to rehydrate while the other vegetables are charring.
When the vegetables are nicely charred, remove the baking sheet from the oven and, when it's cool enough to handle, peel all the garlic; keeping the skins on in this process prevents the cloves for carbonizing. Put all the sauce ingredients into a blender and puree.
Pick all the meat off the turkey legs and reserve. Strain the broth; I do this by setting a fine-meshed sieve over a big bowl. No need to clarify it in this soup. Wipe out your soup bowl and add back the shredded turkey, the strained broth and the contents of the blender bowl. Tip: save back some broth to rinse out every bit of goodness from within the blender bowl. Bring all this to a simmer and let it cook 15 minutes.
When you are ready to serve, stir in the minced herbs and add masa to thicken if you want. Don't add more than a tablespoon at a time, and stir it in well. Serve with rice, little masa dumplings or tortillas.
NOTE: You can find achiote paste in Latin markets, and in the "ethnic" section of large supermarkets. If not, look for annatto, which is the main ingredient in achiote paste.