View more photos in the “Eat Some Goat!” slideshow.

On hot Sunday mornings when the asphalt turns to syrup and the cats begin to pant, the pace is easier than usual on the Eastside, the wait at the bakeries briefer, El Mercado less crowded, the tamale lines at Lilliana’s both shorter and slower moving. The track bed of the Gold Line Extension, although it is set to open any week now, already looks like an abandoned urban ruin, moldering and shimmering in the heat. The brightest-colored wall murals are more oppressive than cheerful. At nine in the morning, when even the rottweilers are dozing in the shade, the only purposeful movement on Fourth Street may be the women wrestling vats of menudo into their cars.

And even on these sleepy midsummer mornings, the most languid place in the area may be Flor del Rio Birrieria Nochistlan, Zac., a restaurant that feels as if it has been occupying its corner since the beginning of time. It is a worn, cheerful place with a mural of Nochistlán’s famous colonial aqueduct, a few framed sketches of a cigarette-smoking wise guy, and enough flowers to make the name of the restaurant at least plausible. The clientele is a little more varied at other times of the week, but Sunday morning is family time, kids gulping candy-colored Jarritos, women sipping coffee through clenched jaws, and unshaven men — it is always men — staring glumly down at the table, nursing their first cold beers of the day. If I didn’t know better, I’d swear that half the customers have decided at the last moment that salvation is closer to hand at Flor del Rio than at church.

There is no menu here, no list of specials, no secret source of carne asada, or machaca, or snook. What Flor del Rio serves, no matter how piteously children may plead for quesadillas or bowlsful of crunchy chips, is birria de chivito Zacatecas-style, young goat roasted and stewed and simmered until it is an animal transformed, a soft, gelatinous sigh drawn from the carcass of a tough-minded billy, an étude in the keys of chile, strong meat and clove. If you’ve ever been to a proper barbecue pit, you’ll recognize the rat-a-tat-tat from the kitchen, the rhythm of razor-sharp cleavers flashing through cooked flesh and bone. Women wander into the kitchen with empty stockpots and Dutch ovens and walk out with gallons of stew.

You can ask for your birria dry, or moistened with the intense goat broth called consomé, or submerged beneath a bowl of soup. You are allowed, I think, to specify whether you want most of your meat to be cut from the haunch, ribs or skull, although results aren’t always guaranteed, and a fairly high percentage of your fellow devotees request boneless birria, which is understandable. I personally dig the bones — more flavor. But birria, like so many of the world’s great dishes, is often consumed as a hangover cure, and at those sensitive times, the last thing some people want to deal with in the morning is anatomical specificity.

With the birria comes both a tongue-warping dried-chile salsa and a squeeze bottle of a thin, vinegary sauce that may be even hotter; both cut through the richness of the meat in different ways. You will want to scatter cilantro and chopped onions over the dish — why not? — and the establishment thoughtfully provides one of the better tortillas in town, thick and broad and almost bulletproof, made to order, with an interior as rich as corn pudding.

A mariachi trio stumbles in, reeking as if they have spent the previous evening at the wrong end of a bottle of tequila, and their leader brandishes his battered guitar like a stringed club, until most of us finally give in, flipping fivers into his brocaded sombrero. It smells as if something has died inside the accordion. But syncopated offbeats ricochet from the guitar, just a millisecond behind in a way that recalls Charlie Christian as much as it does a mariachi band, and from his broken face floats a voice that is rich and clean and pure, beautiful and transformative, following the well-worn contours of a melody older than anybody in the room. Then, as abruptly as they came into Flor del Rio, they are gone. I ask for another basket of tortillas and a cold Especial, and the morning is well on its way.

Flor del Rio, Birrieria Nochistlan, Zacatecas 3201 E. 4th St., Boyle Heights, (323) 268-0319. Open daily, 8 a.m. to 4 p.m. Cash only. Beer. Street parking. Dinner for two, food only, about $20. Recommended — indeed only — dish: birria de chivito.

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