What Christmas main can I serve to a pescatarian with a yeast allergy (vinegar) and mould allergy (cheese and mushrooms)?

I'm hosting Christmas this year and I'm going all out with the turkey and trimmings. However I need to find a separate main for my sister in law. She has always been pescatarian and I usually make her a mushroom wellington. However she recently went to an allergy clinic and they told her to cut out yeast (vinegars, wines) and mould (cheese and mushrooms) for 3 months, and then slowly reintroduce them in small doses to avoid the symptoms she's been having.

Christmas falls in the time when she is cutting out yeast and mould.... Any ideas for what I can serve her as a main? I want to try and make it nice for her, even if she cannot eat the things she'd like

I am not planning on remaking the sides dishes for one person, so whatever I serve needs to compliment the usual roasties, broccoli, carrots, gravy, etc.