Put the masa harina in a bowl and add the warm water, reserving about 1/4 cup of water until you see how much the flour will absorb. Work well with your hands until smooth. Cover with a damp cloth and set aside in a cool spot for about 1 hour. This gives the starch particles time to swell and absorb the moisture thoroughly, giving more flexibility to the dough. Alternatively, make the Dried Corn Tortilla Masa (see recipe).
Work the masa until very soft and smooth, and divide into 12 equal parts. Roll each into a ball about 1 1/4 inches in diameter and cover with a damp cloth while you work. Warm an ungreased comal over medium heat.
Take 1 of the balls and press out gently in a lined tortilla press or by hand to a disk about 3 1/2 inches; it will be thicker than a tortilla, about 1/4-inch. Place carefully on the comal and cook over medium to low heat until the underside is opaque and speckled with brown, about 2 minutes. Turn the sope over and cook on the second side for a further 2 minutes. Remove from the comal and immediately (although you may burn your fingers a little) pinch up the dough around the periphery of the sope, return it to the comal, and cook briefly until the dough is firm and cooked through, about 2 minutes more. Continue with the rest of the balls.
Heat a very small amount of the lard or oil in a skillet and let the sopes heat through for about 1/2 minute on each side. Spread with the bean paste and top liberally with the rest of the ingredients. Serve immediately.
Heat the lard in a heavy 10-inch skillet, add the onion and fry over medium heat without browning, until translucent, about 30 seconds. Gradually add the beans and their broth and continue cooking over fairly high heat, mashing them down to a paste texture, about 10 minutes.
Melt the lard in a small skillet. Skin and crumble the chorizo into the pan, and cook over low heat until the fat has been rendered out. Add the potatoes and chile, if using, and continue cooking over medium heat, scraping the bottom of the skillet from time to time to avoid sticking, until well seasoned, about 8 minutes. Season with salt. Set aside to cool a little before using.
Crush or blend the garlic, chiles and salt to a paste. Gradually add the tomatoes (unpeeled), grinding well after each addition. The sauce should be textured and the skin will never all completely disappear.
Sprinkle the top with the onion and cilantro and serve.
Place the whole chilies on an ungreased griddle over medium heat and turn them from time to time until the flesh is fairly soft; there will be brownish patches on the skin and the color will have faded somewhat. Then, if they are to be ground with other ingredients, chop roughly before blending. Note well: they are to be neither peeled nor seeded.
The whole tomatoes are cooked on a ungreased comal or griddle until they are slightly charred and mushy to guarantee a specially delicious table or cooked sauce. About half the cooks I know then skin the tomatoes, while others, including me, blend them unskinned. While the appearance of the sauce may not be as attractive, the flavor and texture are incomparable. This method of cooking tomatoes is particularly recommended for freezing and storing for the months when tomatoes are not at their best (not a problem in Mexico).
You may want to broil them in a more practical way. Choose a shallow pan in which the tomatoes will just fit in 1 layer, not too large or the juice that is exuded will dry up. (I used to line the pan with foil, but no longer. It is high time that we gradually ease foil out of the kitchen or use it very, very sparingly. The mining of bauxite for the production of aluminum has destroyed far too many tropical forests on this planet.) Place the pan about 2 inches below a heated broiler and broil until the top halves of the tomatoes are soft and the skin is blistered and slightly browned. Turn the tomatoes over and repeat on the other side. The exuded juice will be sweet and syrupy so save it to blend with the tomatoes.
Recipe courtesy Diana Kennedy, From My Mexican Kitchen: Techniques and Ingredients, Clarkson Potter, 2003