Heat 1 tablespoon oil in a pan. Add 1/2 teaspoon mustard seeds and let them crackle.
When they start crackling, add 1/2 teaspoon urad dal & 1/2 teaspoon cumin seeds.
Saute till the urad dal turns golden.
Then add 5 to 6 curry leaves (chopped or kept whole), 1/2 teaspoon finely chopped ginger, 1 dry red chili and 1 green chili (chopped). You can also add a pinch of asafoetida at this step.
Stir and then add 1/4 cup chopped onion. Saute till the onions turn translucent.
Then add the crumbled idli. Also add a bit of salt. Mix very well and saute for a minute or two.
Then switch off the flame. Lastly add 1 to 2 tablespoons chopped coriander leaves. At this step, you can also add some grated coconut.
Mix again and serve idli upma with some coconut chutney or lemon pickle.