Heat the oil in a saucepan. Add the mustard seeds and when they sputter, add the dry red chillies and curry leaves.
Add the onions and saute, stirring, until the onions begin to soften. Add the potatoes and stir to mix.
Add the peanuts and toss together for another two minutes. Add the coconut milk, mix, then add the drained aval and salt to taste.
Toss the aval and the vegetables together well and check for salt. Turn off the heat and add coriander leaves.
To serve, place in bowls and spritz with some lemon juice (in India, the aval is served with a little wedge of lemon on the side). Eat hot.