Christine Morgan ordered a "pensioner's lunch" containing roast lamb at the Clock and Key pub in Trispen, Cornwall, on 11 August 2015.
She became violently ill and died the next day.
The kitchen manager and the company that owns the the pub pleaded guilty to breaching food safety regulations and were fined at Truro Crown Court.
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Diane Burrow, 56, from Crosstown near Bude, managed the kitchen at the pub and was fined Â£750.
The pub owners, Lake Inns and Leisure Limited, was fined Â£20,000. The judge also ordered the company to pay costs of more than Â£23,000.
Mrs Morgan, 71, from Camborne, had visited the pub with her husband and his cousin.
The next day a doctor gave her some anti-sickness medication, but she continued to deteriorate, and died in an ambulance on her way to hospital that night.
Food poisoning was given as the underlying cause of death.
An investigation by Cornwall Council found Mrs Burrow had cooked the lamb four days before serving, exceeding the two days recommended.
She had also failed to ensure the meat was fully cooled before refrigerating it, and to keep records of core temperatures while preparing it.
The food was reheated in a microwave by a member of staff who did not have the required minimum hygiene training.
Tests on the lamb by Public Health England found the same bacteria which was also in Mrs Morgan's stool.
Judge Simon Carr said: "There were systems in place probably in a brightly coloured file on a shelf in the kitchen but this was not sufficient.
"Food safety was considered, but not sufficiently emphasised. There was simply no supervision and control for food handling."
A 71-year-old woman died after getting food poisoning from an incorrectly-prepared pub lunch, a court has heard.