If skirt steak is unavailable, hangar, flank, or other lean, thinly-sliced cuts can be substituted (when in doubt, ask your butcher). If you are fond of fish sauce's funk, use the larger amount (4 tablespoons), if not, stick to 3. If you do not have access to a grill, sear the steak in a large grill pan or cast-iron skillet over high heat; you will likely need to halve the recipe or cook the steak in batches. To make gluten-free, use gluten-free tamari or soy sauce.