Rinse the oxtail under cold water, place in a large pot with 2 liters of water, 1 chopped onion and 1 teaspoon salt. Bring to a boil, then lower heat to a gentle simmer and cook uncovered for 2 to 2.5 hours, skimming foam every 20 minutes, until the meat is fork-tender and almost falling off the bone. Reserve 1.5 liters of the cooking broth and set the oxtail aside.