María Jesús Fernández Calvo, 46, is believed to have ordered a rice and morchella fungi dish at RiFF, which has now been closed to the public until an investigation has revealed the cause of death.
Eighteen other diners including her husband and 12-year-old son were made mildly ill.
Ms Calvo was reportedly celebrating her husband’s birthday. She died on Sunday morning after suffering diarrhoea and vomiting.
Regional health chief Ana Barceló sent her condolences to the family and said that an investigation was underway.
“We’ve conducted a primary inspection of the establishment and everything appears to be normal,” she told Las Provincias newspaper. “Analytical tests will now be carried out on the food products.”
Samples of ingredients from the restaurant have been sent to Spain’s National Institute of Toxicology and Forensic Science.
Morchella, or true morel, is popular in early spring. They are normally dried out and then well-cooked in order to destroy any presence of the toxin hydrazine.
However, there are several lookalike species known as false morels, which can be lethal.
Chef and owner of the restaurant Bernd Knöller said in a statement: “Regardless of the reason that may have caused this situation, I want to convey my deep regret for what happened, hoping that soon all these facts can be clarified.”
He added: “I have made the decision that the restaurant remains closed until the causes of what happened are established and the activity can be resumed with full guarantees for both staff and all customers.”
Mr Knöller said he is cooperating fully with the authorities.
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