To a kadai, add oil. Wait for it to heat up and add the mustard seeds, urad dal. Once the mustard pops and urad dal browns, add sliced onions and curry leaves.
Saute for 30 seconds, then add the green chilli. Add a little salt, hing and let the onions soften a bit.
Now, add the tomato ( note, the tomato is a lot lesser than the onions in this dish), followed by the sambar powder, turmeric, red chilli and salt.
Saute for another 30 seconds and add the water. We need to form a slightly thick gravy that will cook the onions, tomatoes and spices.
To this masala, add chopped boiled potatoes. Mix well and then add the frozen peas.
Add salt to taste, reduce flame and keep closed for 2-3 minutes.
Open, add coriander leaves and cook on a low flame for another 2 minutes. There must still be some moisture left in the curry.
Turn the flame off, squeeze a little lemon juice and serve!