Melt the lard or other fat in a medium-size skillet set over medium heat. For a milder dish, seed the chiles then chop them finely and add to the skillet, along with the onion and tomato. Cook, stirring frequently, until the onion has softened but is not brown, about 5 minutes. Reduce the heat to medium-low.
Add the shrimp to the skillet, letting the shrimp cook until they're almost done through, about three minutes. Add a big handful of chopped cilantro and a squeeze of fresh lime juice, then scoop into a warm tortilla.