Pour the vinegar and sugar into a medium bowl. Add a pinch of salt and whisk together.
Quarter the apples. Slice out the cores and discard them. Slice the apples as thinly as possible. Place them into the bowl with the cider vinegar mixture and turn them a few times to coat them. Set to one side to pickle.
Tip the buckwheat into a small saucepan. Place the pan on the heat and toast the buckwheat for a few mins till darkened slightly and nutty smelling. Carefully pour in 150ml boiling water and simmer on a low heat for 10 mins till the buckwheat is tender. Drain any water off and set aside to cool.
Remove any tatty leaves from the cabbage and cut it in half (see our tip for what to do with the rest). Finely shred the cabbage half as thinly as possible, including the juicy core. Place into a large bowl.
Pop a small frying pan on the hob for a few mins. Tip in the pumpkin seeds and toast for a 2-3 mins till thes start popping. Tip into a small bowl.
Finely chop the mint leaves.
When ready to serve, tip the buckwheat into the bowl with the cabbage. Pour over the apples and their pickling liquid. Drizzle in 2 tbsp olive oil. Add most of the blackberries and half of the mint. Toss everything together with a pinch of salt and pepper.
Divide the cabbage between a couple of plates. Top with the remaining blackberries and mint and scatter over the toasted pumpkin seeds to serve.