Place the sliced onions in a bowl, add some salt and cover with warm water. Let rest for about 10 minutes, drain and rinse well with cold water.
Place the cooked langostinos in a glass or plastic bowl; add the lime juice, washed onion slices, hot peppers, cilantro sprigs, garlic clove, and salt. Cover and let marinate for a couple of hours in the refrigerator.
Remove the hot peppers, the cilantro springs and the garlic clove.
Add the cilantro and olive oil to the marinated langostinos and onions, mix well. Taste and add additional salt if needed.