Preheat a grill pan over medium-high heat. Whisk together 1 tablespoon olive oil and the chili powder, then rub all over the steak; season with salt and pepper. Add to the grill pan and cook, turning once, until grill marks appear, about 5 minutes for medium-rare. Transfer to a cutting board and let rest for 10 minutes before thinly slicing across the grain.
Meanwhile, in a medium bowl, combine the jalapenos, lime peel, lime juice and remaining 3 tablespoons olive oil; season with salt and pepper. Add the lettuce, avocados and onion and toss; season with salt and pepper.
Divide half of the avocado salad among the taco shells, then top with the meat and the remaining avocado salad.