Preheat the oven to 180°C/350°F/gas 4. Place the grapes on a baking tray, finely grate over the zest from ½ the lemon, drizzle with a little oil and season with sea salt and black pepper.
Place in the hot oven for 15 minutes, then add the walnuts and roast for a further 5 to 10 minutes, or until the grapes are soft and caramelised and the walnuts are golden.
Meanwhile, make the dressing. Place the mustard and yoghurt into a bowl and whisk well. Pick, finely chop and add the tarragon leaves, squeeze in the lemon juice and mix well, then season to taste.
Trim and chop the celery into 1cm chunks, slice the apple into large matchsticks, then place into a large bowl. Discard the tatty outer lettuce leaves, then roughly chop and add to the bowl. Pick the grapes off their stalks and add to the bowl. Drizzle over the yoghurt dressing and toss well. Place onto a platter, roughly chop and sprinkle over the walnuts, then serve.