Peel the skin and cut it in half and then into quarters. This is the time to take a taste test of the flesh and seeds separately. If the seeds taste bitter, then use a spoon to scoop them out. Then chop it into bite size pieces.
Make the pappu:
Wash and drain toor or masoor dal thoroughly. Combine the chopped cuke along with lentils, onion & tomato (if using), green chilies, turmeric, salt and water. Give a stir and lock the lid. Put the steam valve to 'sealing' and cook for 8 minutes on High Pressure/ Manual for toor dal or 6 minutes on High pressure for masoor dal.
When the timer sounds, let the pressure release naturally for 15 minutes, quick release the remainder. Open the lid and stir everything together.
If using the pressure cooker, then cook for 3~4 whistles.
Add tadka (tempering):
In a small saucepan, heat oil, add mustard and cumin seeds, dry red chili and cook till they start to pop. Then add curry leaves, asafetida and cook for another 30 seconds. Add the tempering to the cooked dal. Mix well and serve hot with rice and ghee.
Notes
This Andhra style cucumber dal recipe is traditionally made with dosakaya or yellow round/ oblong cukes. But you can also use either English or American style cucumbers.
If using other type of cukes or if the lemon cuke is not sour enough, then add lemon juice or tamarind paste to give flavor to the dal.
Make sure to take the cuke before using. There are chances that the flesh or seeds might be bitter.
Leftover dal can be stored in an airtight container for up to 2 days.
Omit onion if you are not a big fan.
Add 1~2 crushed garlic cloves for extra flavor along with the veggies in the Instant pot.