To make the taco shells, mix masa harina, water, and salt together until it forms into a soft ball. If it’s too dry, add a bit more water a tablespoon at a time.
Divide dough into 16 equal balls, and press out with a tortilla press or roll out with a rolling pin. Keep pressed discs covered with a damp cloth.
Heat up 2 inches of oil in a pot or skillet to 350°F. Gently place a disc in the hot oil and it should immediately start to puff. After 5 seconds, with a spatula, make an indention in the center so it forms a V shape.
Gently cook the shell on each side until light brown and crispy (about 20 seconds for each side).Remove from oil and drain on a paper towel.
To make the tacos, for the meat, heat the oil in skillet on medium heat. Throw in onions and jalapeños, and cook for about 10 minutes. Add garlic, and cook for 1 more minute.
Stir in the beef, chili powder, cumin, oregano, salt, pepper, cayenne, cilantro, and salsa until well combined. While stirring occasionally, cook the meat until meat it’s browned, about 15-20 minutes. Taste and adjust seasonings, then stir in the lime juice.
To serve, stuff into shells, then top with shredded cheese, lettuce, and diced tomatoes.