off-flavour

Hi everyone, I made a MJ Saison kit last month (first brew), it’s been conditioning in bottle for 3 weeks and have had the odd taste, before bottling, a week in, and after 3 weeks... and it tasted pretty rubbish every time... I’m having trouble describing the flavour according to lists of “off...

Brewed a dortmunder export recipe a few months ago. The taste was fine from the fermenter but since it was bottled a strong cider/green apple taste sprung up. Any ideas how to prevent this in the future or how to remedy it. Used two packs of Bohemian lager yeast by the way.