If you don't have a pressure cooker, you could cook this in a large pot, make sure veggies and lentils have cooked well.
Drumsticks, ash gourd and raw plantain are available in most Indian and some Asian superstores.
Raw plantain is just optional, so you don't have to add it.
Make sure you add asafetida, as this gives a nice aroma and taste to the sambar.
If you are using concentrated tamarind paste,use less amount may be 1 tsp or so.
Coconut oil makes the sambar taste better, if you don't have you could use vegetable oil.
While making the seasoning, add the fenugreek seeds only at the end and don't over fry it as it would impart a bitter taste to the sambar.
Adjust the spice and tamarind according to your needs.
Sambar would turn thick when kept longer, while heating it add enough water to thin it down.