Preheat the oven to 180ºC. Place the grapes on a baking tray, finely grate over the zest from 1/2 a lemon, drizzle with a little oil and season with salt and pepper.
Place in the hot oven for 15 minutes, or until the grapes are soft and starting to burst.
Meanwhile, make the dressing. Place the mustard and yoghurt into a bowl and whisk well. Add the tarragon, squeeze in the lemon juice and mix well, then season to taste.
Slice the apple into thin rounds, then place into a large bowl of iced water with 1 lemon squeezed in. Shred the brussels sprouts with a mandolin and add to the bowl.
Drain the water off and place the brussels and apples onto a large serving platter.
Drizzle over the yoghurt dressing and toss well. Place the grapes onto the serving platter and sprinkle over the walnuts, then serve.