Pat the beef shank dry and season lightly with the salt and a few grinds of black pepper. Mole de olla is usually more about the broth than heavy browning, but if you want a slightly deeper beef note, lightly brown the shank first. Heat the tablespoon of oil in a heavy pot or Dutch oven over medium-high heat and sear the beef for 2–3 minutes per side until lightly colored, not deeply crusted. Add the halved onion, garlic, bay leaves, peppercorns, and cilantro stems if using. Pour in the 3 litres of water. Bring to a gentle boil, then immediately reduce to a low simmer. Skim any foam and impurities from the surface several times during the first 20–30 minutes. Simmer partially covered for about 1 hour 30 minutes, or until the beef is well on its way to becoming tender. The broth should look clear and smell deeply beefy, not cloudy and harsh. A gentle simmer matters — a hard boil emulsifies fat and impurities into the broth and muddies the final result. Skimming repeatedly during this early window matters directly, since the most concentrated release of foam and impurities happens in those first minutes of simmering — staying attentive through that period is what sets the broth's clarity for the many hours of cooking still ahead.

While the broth simmers, wipe the dried chiles clean. Remove stems and seeds from the guajillos and anchos. Toast the chiles briefly in a dry skillet over medium-low heat for about 10–15 seconds per side, just until fragrant and slightly flexible. Do not let them blacken or they will turn bitter. Transfer them to a bowl and cover with hot water. Soak for 15–20 minutes until softened. In the same skillet, roast the tomatoes, the 60g onion, and the 2 garlic cloves until softened and lightly charred in spots. Drain the softened chiles and place them in a blender with the roasted tomatoes, roasted onion, roasted garlic, cumin, and Mexican oregano. Add about 300–500ml of hot broth from the pot and blend until extremely smooth. For the most refined broth, strain this blended chile base through a fine sieve. The puree should be smooth, brick-red, and fragrant, with no tough chile skin pieces left. Straining is worth it here because mole de olla should have body from the chile base, but still feel like a broth rather than a rough stew puree. Using hot broth from the simmering pot itself to help the blender move, rather than plain water, means the chile base carries genuine beefy depth built directly into it from this very first blending stage.

Once the beef has simmered for about 1 hour 30 minutes and the broth is flavorful, remove and discard the onion halves, garlic, bay leaves, peppercorns, and cilantro stems if you can easily retrieve them. Pour the strained chile base into the pot and stir well. Simmer for another 20 minutes so the broth and chile mixture fully integrate. Taste the broth and add more salt as needed. After this simmer, the broth should taste rounded and cohesive — beef first, then the gentle fruitiness of guajillo and the darker depth of ancho. It should not taste like raw blended chile dropped into stock. This further simmer after combining is what actually integrates the chile base into the broth rather than leaving the two tasting like separate components sharing the same pot — that cohesion takes real, if brief, cooking time to develop.

Add the corn pieces and potato wedges to the pot. Simmer for 10 minutes. Then add the chayote and the xoconostle if using. Continue simmering for another 10–12 minutes. This staged addition matters because corn and potatoes need more time than zucchini and green beans. At this stage the beef should be nearly tender and the broth should be a clear deep red. The potatoes should be starting to soften at the edges, but not yet fragile. Working through the vegetables in genuine stages, rather than adding everything together at once, respects each ingredient's own real cooking time — corn and potato need meaningfully longer in the broth than the more delicate vegetables still to come, and staggering the additions is the only way to bring every vegetable to properly tender at roughly the same final moment.

Add the zucchini, green beans, and the epazote. Simmer for another 8–10 minutes, just until the zucchini is tender, the green beans are cooked but still green, and the beef shank is fully tender. Taste again and adjust salt if needed. If too much liquid has reduced, add a little hot water. Mole de olla should be brothy and generous, not tight and stew-like. If there is excess fat on the surface, skim lightly — but leave enough to keep the broth rich and glossy. The final broth should be vivid red and aromatic, with the herbal note of epazote present but not dominant. The vegetables should all be distinct: potatoes soft, corn sweet and juicy, chayote tender, zucchini intact, and green beans cooked but not drab. Adding the epazote only at this final stage, alongside the quickest-cooking vegetables, protects its distinctive herbal character from being cooked out over hours of simmering the way it would if added earlier alongside the beef.

Place one large piece of beef shank in each bowl. Ladle over plenty of broth and distribute the vegetables evenly so each serving gets corn, potato, chayote, zucchini, and green beans. Top each bowl with a small piece of epazote if you like the visual, and serve with lime wedges on the side. Warm corn tortillas are the most classical accompaniment. The bowl should look abundant but not crowded. The broth is just as important as the solids, so serve generously with plenty of red broth. Ladling generous broth into every bowl, rather than favouring the solids, respects this dish's genuine identity as a brothy stew rather than a thick, sauce-heavy one — the broth itself carries much of what makes mole de olla distinctive.