How to Make It
In a saucepan, combine the water, onion, cilantro stems, garlic, jalapeño, crushed coriander, whole peppercorns and 1/2 teaspoon of salt and bring to a boil. Add the meat and simmer over moderately low heat until tender, about 25 minutes. Transfer the meat to a plate and let cool slightly; thinly slice the meat across the grain. Reserve the broth if desired.
Meanwhile, in a small skillet, toast the whole coriander seeds until fragrant, about 3 minutes. Transfer to a spice grinder or mortar and grind to a powder. Put the ground coriander in a small bowl and whisk in the lime juice, 1/2 teaspoon of salt and the freshly ground pepper. Whisk in the oil until combined.
Line a platter with the romaine. Toss the sliced meat with 1/3 cup of the dressing and the chopped cilantro. Mound the meat on the lettuce and garnish with the avocado, radishes and scallions. Drizzle the remaining dressing on top and serve with warm tortillas and hot sauce.