FOR MEDU WADAS: After soaking urad dal, drain the liquid.
Grind the dal with green chilies, curry leaves, and ginger to a fine paste (in 3 batches).
Remove to a bowl and add sliced chilies, coriander leaves, and Salt to taste. Mix well.
Heat oil in a deep wok/kadhai. Take some clean water in a bowl.
Keep a wide flat katori handy.
Wet the base of katori, take a large lemon-sized batter and place it on the base of katori. With wet finger, make a hole in the center.
Drop the batter in the medium hot oil and hold katori till the batter loosens up and the katori can be separated easily.
Fry both sides till golden brown.
FOR SAMBAR: Soak dal for at least 3 hours in very warm water.
Wash dal and pressure cook till soft. Once pressure is gone, remove container of dal and churn it till it is smooth.
Dry roast all ingredients under Masala except grated coconut till lightly browned. Add 3-4 drops oil and stir. Switch off gas. Add grated coconut and sauté till its lightly colored. Cool and grind in food processor till fine powder is formed.
Soak tamarind in warm water for 5-10 minutes and squeeze out the pulp discarding the pith.
Take a deep pot and add dal to it. Add water according to sambar consistency (not too watery). Add the roasted masala to the dal and all the vegetables except drumbsticks.
Boil drumbsticks separately.
Bring the dal to a boil. Cover and cook till vegetables are done.
Add tamarind pulp and cook for some more time till tamarind flavor is blended in the dal.
Prepare the tempering. In another pan, heat oil. Add mustard seeds, hing, red chilies, and curry leaves. Sauté. Add fenugreek seeds. Stir for few seconds and add the tempering to the dal. Quickly place a lid as soon as the tempering is added to infuse flavors in the dal.
Keep simmering on very low heat for additional 5-10 minutes on low heat.