Preheat the oven to 325 degrees . Arrange the taco shells on a baking sheet and toast until golden, 5 to 8 minutes. Cool slightly, then break into bite-size pieces.
Coat the steak in the oil; season with salt and pepper. In a small bowl, combine the spices; rub all over the steak.
Heat a cast-iron skillet or grill pan over medium-high.
Add the steak to the skillet; tent with foil. Cook the steak, turning once, about 5 minutes per side for medium-rare. During the last 3 or 4 minutes of cooking, add the limes to the skillet, cut sides down. Transfer the meat to a cutting board. Let rest 5 to 10 minutes. Slice thinly against the grain; douse with the juice of the grilled lime halves.
In a medium bowl, combine the tomatoes, onion, chopped jalapeno and cilantro to make pico de gallo; season with salt.
Arrange the romaine on a platter or plates. Top with taco shell pieces, cheese, steak, pico de gallo, radishes and pickled jalapeno slices. Serve with the hot sauce.