This is probably the easiest fermented thing to make, and it adds so much goodness to your cooking.

Here’s the recipe:
Take 2 tablespoons of créme fraîche and place it in a clean* glass pint jar (450ml).  Put around 2 tablespoons of fresh pouring cream (this needs to be 100% cream, no additives, no preservatives) into the jar, and stir with a spoon to mix it in.  Add a bit more fresh cream and mix it again.  Then fill the jar up completely with cream and stir again.  Leave to sit at room temperature for 12 to 24 hours, until thickened and slightly sour, then store in the fridge.

This is wonderful with all kinds of fruit crumbles, fresh fruits, and anywhere that you would use sour cream.  I like to add it to frittatas and quiches for added depth of flavour.

Créme fraiche will keep in the fridge for around 2 weeks.

Jars of créme fraîche to use as a starter culture is usually found lurking in the refrigerated section of snooty grocery shops, usually with the other cream, but sometimes with the specialty cheeses and smallgoods.  Just try to ignore the gaggle of cackling boomer hags and push past them as you find your way towards the fridge.  Once you have your first jar of it, you can keep culturing your own créme fraîche in the safe surroundings of your home forever.  The ingredients on it should just be cream and cultures.

Alternatively, you can use cultured buttermilk, or milk kefir, or a pinch of flora danica (aromatic mesophilic) cheesemaking culture.

*I heat sterilise all my jars by taking the lids off and putting them in a warm oven (110ºc or 230ºf).  I bring a pot of water to the boil, then boil the lids for 30 seconds, tip the water out, put the lid back on and allow to cool, then when the jars have heated up enough I turn the oven off and put the lids in the oven to dry.  For a longer explanation see this post.