Heat oil in a pressure cooker, add urad dal, mustard & cumin seeds and curry leaves. Once the seeds start to splutter, add the onions and sauté until lightly browned around the edges, takes about 4-5 minutes.
Add turmeric and ginger+garlic paste at this point; mix well and cook for 1 minute.
Add tomato puree and cook for 3-4 minutes.
Next add ground cashews, red chili powder, curry powder, salt and chopped chayote. Mix well.
Add ½ cup of milk (if using) or water, no need to add too much water since chayote gives out moisture while cooking.
Close the lid and pressure cook on medium flame for 2 whistles.
Once the pressure drops, open the lid and serve hot with rice or rotis.