How to Make It
Cut onion lengthwise into quarters. Chop 3 of the onion quarters, and set aside. Heat 2 teaspoons lard in a skillet over medium. Add onion wedge and garlic; cook, stirring occasionally, until onion wedge blisters and turns deep golden brown, 6 to 7 minutes. Remove and discard onion wedge and garlic. Carefully add beans and 1/4 cup warm water to skillet (mixture will splatter). Using a potato masher or the bottom of a heatproof cup, roughly mash beans. Add additional water as needed, 2 teaspoons at a time, mashing after each addition, until beans are chunky-smooth. Stir in 1 1/2 teaspoons salt. Reduce heat to medium-low; cover and cook, stirring occasionally, until beans are slightly thickened and creamy, about 5 minutes, adding water as needed if skillet looks dry. Remove from heat; season with salt to taste. Transfer beans to a bowl, and set aside.
Melt 1 tablespoon lard in a small saucepan over medium-high; remove from heat. Place masa harina in a deep bowl; add 1 cup warm water, melted lard, and remaining 1 teaspoon salt. Knead until a cohesive dough forms, about 2 minutes. Add remaining 1/4 cup warm water, and knead into dough. To test whether dough is moist enough, break off a piece, roll it into a ball, and flatten it. If dough edges crack, add additional water, 1 teaspoon at a time, until edges no longer crack.
Remove a small piece of dough from prepared masa; roll into a 2-inch ball (just larger than a golf ball). Cover dough in bowl with a damp kitchen towel. Using the palms of your hands, pat dough ball into a 4-inch disk (about 1/4 inch thick). Alternatively, press lightly with a tortilla press lined with sheets of plastic. (A ziplock plastic bag works best.) Invert dough disk into your hand. (If it sticks to your hand, dough is too wet; add additional masa harina as needed to correct the texture.)
Spread 1 level tablespoon mashed beans down center of dough disk. Fold both sides of disk in toward center, enclosing the filling like an empanada. Pinch seams closed. Place filled tlacoyo, seam side up, on a tortilla press lined with sheets of plastic; flatten into a 1/4-inch-thick oval-shaped patty. (You can also flatten tlacoyo using a rolling pin.) Repeat Steps 3 and 4 with remaining dough and remaining mashed beans to make a total of 8 tlacoyos.
Melt 1 teaspoon lard in a 10-inch cast-iron skillet or comal (Mexican griddle) over medium. Add 4 tlacoyos to skillet. Cook until sides start to dry slightly and begin to brown, 3 to 4 minutes. Flip tlacoyos. (If you don’t see golden-brown freckles on tlacoyos, increase heat slightly; if you see burned spots, reduce heat slightly.) Cook until both sides of tlacoyos are dark-golden in spots and crisp, about 4 minutes. Remove from skillet; place inside a clean kitchen towel, and wrap tightly to keep warm. Repeat procedure with remaining 1 teaspoon lard and remaining 4 tlacoyos. Serve tlacoyos warm; top evenly with nopales, if using; cheese; cilantro; salsa; and reserved chopped onion.