Chilli-Spiced Steaks with Avocado Sauce

In the Yucatán Peninsula of Mexico, thin, flat strips of pounded steak are marinated in a citrus-flavoured chillied paste, then quickly grilled over an open fire. (In the Yucatán, the meat of choice would probably be venison or other game; lamb is also good.) Spring onions are often grilled alongside, becoming sweet inside and charred on the outside. Guacamole makes a gentle yet tangy counterpoint - this guacamole is different in that it is made with pasilla chilli (that is, mild chilli pow