In a large bowl or casserole dish, whisk together the zest and juice of the limes, tequila, oil and a hefty pinch of salt. Add the meat the marinade, toss to coat and refrigerate for at least 1 hour and up to overnight.
To cook, preheat an outdoor grill or grill pan over high. Lightly brush the grill with a little oil and grill the steak until seared, 5-6 minutes per side for medium. Let the steak rest on a cutting board for a few minutes before slicing.