For rice vinegar dressing, simmer stock over medium heat until reduced to ¼ cup (5-6 minutes). Remove from heat, add remaining ingredients, whisk to combine and set aside.
Cook noodles according to packet instructions. Drain, resfresh in cold water, drain again, coarsely chop and set aside.
Halve eggplant lengthways, then quarter each half lengthways. Place in a bowl, cover with cold water, stand for 10 minutes to remove bitterness, then drain. Place in a steamer basket over a saucepan of simmering water and steam until soft but still holding their shape (about 15 minutes). Remove from steamer and cool. Gently squeeze excess water from eggplant, then place on a serving plate. Place noodles over eggplant, drizzle with dressing, scatter with coriander sprigs and serve immediately.