Mrs. Scorbic is the cook in Lancre Castle.
It is likely that she assumed the position of Castle Cook when her predecessor had a gibbering nervous breakdown, faced with Duke Felmet's demands for porridge for breakfast, as opposed to a procession of formerly living things with apples stuffed in their mouths. Finding no way of plausibly stuffing an apple into a bowl of baked porridge without it looking ridiculous, the former cook ceased his employment at the castle...
She considers boiling is the secret of good cooking, and her interpretation of "herbs" is not one that Magrat Garlick shares.
But even as Queen, to gain the courage to tell her employee what she would like to see cooked, Magrat has to resort to wearing the helmet of Queen Ynci and pointing a crossbow at her. Still, it'll be the last time the unfortunate phrase chit of a girl is ever used by the Cook to describe her Queen and employer. At least, where Magrat might hear it.
When the Vampyres take over Lancre, Mrs. Scorbic is reduced to uncharacteristic docile servility by dint of the repeated application of fangs to her jugular. In this soporific state, she loses her usual acidic demeanour and becomes docile and rather more pleasant than usual.
Her name seems to be a derivative of ascorbic acid (Vitamin C), whose name means, effectively, anti-scorbutus; scorbutus being more commonly known as scurvy. So she is Mrs Scurvy - a probable result of her cooking.
Her temperament is also well described by the word 'ascerbic'.