What it is
Asiento (literally “what settles” or “sediment”) is the unrefined brown residue that collects at the bottom of the pot when pork lard is rendered for carnitas. As the white fat slowly clarifies and rises, browned protein solids, caramelized milk sugars, and concentrated meat drippings sink to the bottom; once the clear lard is poured off, what remains is a thick, dark-tan, intensely savory paste streaked with crispy bits — the asiento (Kennedy 2010; Trilling 1999). It is, functionally, the pork-fat equivalent of beurre noisette plus fond, and the byproduct of every traditional carnitas operation.
Why it is structural to the tlayuda
The defining first move in building a Oaxacan tlayuda is smearing a thin layer of asiento directly onto the giant (30+ cm) toasted tortilla — before the beans, before the quesillo, before the meat. The asiento waterproofs the tortilla against the wet toppings, perfumes the whole construction with rendered pork, and replaces the role that lard, butter, oil, or refried beans alone would play in other Mexican antojitos. Lopez (2019) calls it “the soul of the tlayuda”; Esparza (2018) writes that a tlayuda built without asiento “isn’t a tlayuda — it’s a giant tostada.” It also appears as a base spread for memelas and as a flavor-builder folded into pots of beans and amarillo moles.
Regional name variants
In some Oaxacan and Pueblan villages the same residue is called asiento de chicharrón (when scraped from the chicharrón pot rather than the carnitas pot), and Kennedy (2010) records the term manteca asentada in northern Oaxacan towns. The word asiento without qualifier is unambiguous in Oaxaca-city and Valles Centrales usage.
How it is sold commercially in LA
Because asiento is a byproduct, it is rarely produced in industrial scale; what reaches LA is artisanal. Guelaguetza Foods retails small glass jars or plastic tubs (typically 8–16 oz) of asiento, distributed through their Olympic Blvd store and select Pico-Union/Koreatown bodegas. Oaxacan importers occasionally bring in tinned asiento from Etla and Tlacolula producers. Mercado Olympic vendors sell house-rendered asiento by the cup on weekends, often alongside fresh chicharrón. Sabores Oaxaqueños (3337 W 8th St) renders its own from in-house carnitas operations and uses it on every tlayuda. Home cooks in LA who render carnitas at home should reserve the dark bottom layer rather than discarding it — it is asiento.