These tostadas channel the spirit of one of Oaxaca's most beloved street foods--the iconic tostadas de salchicha, made with the smoky, umami-packed, bright-red sausage from the city of Ejutla de Crespo. In this easy at-home version, store-bought chorizo (or spicy Italian sausage as a great stand-in) is crumbled and crisped, then piled onto golden, crunchy tostadas layered with a thick, bright salsa de guacamole--avocado whipped with fresh lime j…