Saambar for Vadai. ( oyu do not need any vegetable other than onion).

For 4-5 persons

Ingredients

one LARGE onion (red preferably);   one LARGE tomato;  a sprig of
curry leaves; 
three fourths of a tsp. tamarind paste
salt, oil, mustard seeds, methi, etc. 

Roast together: 1 tablespoon( or 3 teaspoons) coriander seeds,
                one heaped teasp. whole black pepper
                two teasp. jeera
                one table spoon sesame seeds
                a few curry leaves
                half teasp. methi
                10 red chillies
               
Toor dal or masoor dal-- 1 cup, boiled well, and mashed with a
ladle.
Turmeric powder
Cilantro


Method:
Heat a tablespoon of oil. Add mustard seeds (half teaspoon), methi (one
fourth of a teaspoon), and a little turmeric powder. When they
crackle, add curry leaves. Add onions and some salt, fry for 10
minutes in medium high heat. Then, add 4 to 5 cups of water and
the tamarind paste. Stir now and then, and let it come to a
vigorous boil. Turn the heat down to medium, and let it stay
there. Add chopped tomatoes. Wait till the vegetables get cooked
completely; while waiting, grind the roasted stuff together
either dry, or with half a cup of water; once well ground, add
this to the simmering soup on stove. Mix well, check for salt.
Increase the heat. When it boils well, add the cooked dal, and
cilantro. Keep it covered until ready to use.