With a large, heavy, sharp knife or Chinese cleaver, cut the meat into chunks and chop fairly fine, not quite as fine as ground meat. The pieces should still be visible as tiny dice. Set aside.

Peel the potato; peel and pit the chayote. Cut into fine (1/4-inch) dice, place in a medium-sized saucepan, and add water to cover. Bring to a boil over high heat and cook for 3 minutes. Drain and set aside.

In a large skillet, heat the oil to rippling over medium-high heat. Add the onion and garlic and cook until the onion is translucent, about 3 minutes. Add the tomato and cook another 5 minutes to evaporate most of the juices. Stir in the chopped meat, cumin, pepper, and dried oregano. Reduce the heat to medium and cook, stirring, for about 3 minutes, or until the beef has changed color. Stir in the diced potato and chayote with the herbs. Dissolve the masa in the stock along with the optional achiote paste and add to the pan. Cook, stirring occasionally, for 3 minutes or until the liquid has formed a medium-thick gravy.