How to Make It
Heat the oil in a small saucepan. Add the onion and garlic and cook over moderate. heat, stirring, until softened, about 1 minute. Reduce the heat to low. Add the guava paste and water and cook, stirring, until the paste has dissolved, about 5 minutes. Stir in the soy sauce, ketchup and cayenne. Season with salt.
Set the pork tenderloins in a 9-by-13-inch glass baking dish and brush them all over with half of the guava glaze.
Light a fire or heat a grill pan over moderate heat. Grill the pork, turning and brushing with the remaining glaze, until browned on all sides and cooked through, about 15 minutes. Transfer the pork to a cutting board, cover loosely with foil and let stand for 10 minutes.
In a bowl, mix the cilantro, oil, vinegar, onion, chives, jalapeños and garlic. Season with salt.
Thinly slice the pork. Garnish with cilantro sprigs and serve with the salsa.
Guava paste is available in the Latin section of most supermarkets.