Roughly chop the whites of the spring onions and add to a food processor, along with the shallots and garlic, then blitz to a fine paste.
Add a lug of rapeseed oil and the paste to a large pan over a low heat. Fry for around 5 to 10 minutes to really get the flavours going.
Meanwhile, peel, core and roughly chop the apples, then add to the food processor along with the thyme, lovage and tarragon leaves, celery salt, ginger and nutmeg. Blitz well until smooth and add to the pan, with the cassia or cinnamon stick, bay and cloves. Cook for a further minute, then stir in the sugar and continue frying for a further 3 minutes, or until the sugar starts to dissolve and you have a thick brown paste.
Pour in 285ml cold water and cook for a further 2 minutes, before adding all the remaining ingredients. Stir well and bring to the boil, then reduce the heat to medium and simmer for 10 to 15 minutes, or until thickened slightly.
Pour the sauce through a sieve into a large bowl, then pass through the sieve again so you get a really smooth sauce. Return the sauce to the pan and simmer over a low heat for around 10 minutes, until reduced by about a quarter (you want to end up with around 500ml, 1 serving=1 tablespoon).
Leave to cool completely, then either get marinating straight away, or divide between airtight, sterilised jars and it should keep for up to 6 months.