Tip the buckwheat into a pan and cover with 300ml boiling water. Return to the boil, then reduce to a simmer and cover for 15 mins, till tender. Drain through a sieve, and rinse well with cold water to cool quickly. Shake off any excess water and tip into a large bowl.
While the barley cooks, heat your oven to 200°C/Fan 180°C/Gas 6. Trim the runner beans and slice into rough 3-4cm pieces. Tip into a roasting tin and toss with 1 tbsp oil and a pinch of salt and pepper. Slide into the oven and roast for 10 mins, turning halfway, till tender and slightly caramelised on the outside. Add to the buckwheat in the bowl.
Halve the plums and scoop out the stones with a spoon. Slice into wedges. Drain the mozzarella and tear into rough pieces. Pick the basil leaves and roughly chop them. Add all three to the bowl with the buckwheat and beans.
Halve the lemon and squeeze the juice into the salad. Pour over 1 tbsp oil and sprinkle in a generous pinch of salt and pepper. Toss everything together well.
Heap the salad onto a big platter or a couple of plates. Finish with a scattering of rocket leaves
and serve straight away.
A haven for late summer fruit^
Fancy using something other than plums for this salad? A whole host of other late summer fruits are ready and waiting as suitable understudys. Peaches, pears, berries and figs will also work a treat.