Heat your oven to 200°C/Fan 180°C/Gas 6. Slice the aubergines into around 8 wedges each. Place them in a roasting tin and brush with 1 tsp oil. Season with a pinch of salt. Cover with foil and roast for 35-40 mins till soft.
While the aubergines roast, warm 1 tsp oil in a frying pan. Add the buckwheat and fry for 2 mins till golden and crisp. Tip out onto a plate lined with kitchen paper to drain. Set aside.
Use a small, sharp knife to slice the skin from the orange. Segment the flesh over a mixing bowl to catch the juice. Put the segments to one side. Squeeze any extra juice from the flesh clinging to the orange skin into the bowl.
Whisk the tahini into the orange juice with the vinegar and 2 tsp honey to make a dressing.
Arrange the salad leaves on two plates. Tip with the orange segments and roast aubergine wedges. Crumble half the pack of feta over the salad and scatter over the buckwheat. Drizzle over the dressing and serve.