To make the dressing: In a large bowl, whisk together the lime juice, low sodium soy sauce, splenda, and chili pepper. Measure out 1 tablespoon and set aside.
To make the salad: Season the steak generously on both sides with salt and pepper. Grill on a grill or in a lightly oiled stovetop grill pan over medium-high heat, 4 to 5 minutes per side (steak is best rare to medium and still pink in the center). Set aside and keep warm.
To the bowl with the dressing, add the radishes, onion, cucumber, mint, basil and lettuce, and toss. Arrange on a large platter. Thinly slice the steak across the grain and toss with the reserved 1 tablespoon dressing. Arrange on top of the salad.