There are  many different varieties of sambhar, but the araichu vitta sambhar would definitely occupy the top rank amongst them. I am posting the basic recipe for this sambhar here. Other ingredients can be added to suit one's preference. Ideally eaten with steaming hot rice and a liberal amount of home made ghee. With potato onion curry and fried appalam as accompaniments, the taste is heavenly.


Ingredients

Toor dal                                                             1/2 cup
Small onions (peeled)                                    1 cup  
(other veg like white pumpkin, red
pumpkin, carrot, beans, etc can be 
used) 
Red chillies                                        8
Coriander seeds                               11/2 tsp
Bengal gram dal                               1 tsp
Tamarind                                            lemon sized ball
Oil                                                        2 tblsp
Fenugreek seeds                             1/4 tsp
Mustard                                               1/2 tsp
Salt                                                       to taste
Turmeric powder                              1/4 tsp
Grated coconut                                  3 tsp
Asofetida                                             1/4 tsp
Curry leaves                                       a few
Coriander leaves                                for garnish


Method

Pressure cook the toor dal until soft. Mash it well and keep aside.
Heat 1 tblsp of oil in a pan. Add the coriander seeds, bengal gram dal and 6 red chillies and fry till it gives out a nice aroma.
Add the grated coconut and fry for another minute. Cool and grind this into a fine paste adding water.
Soak the tamarind in around 2 cups of hot water for a few minutes and extract the juice from the tamarind.
Heat the remaining oil in a pan, add the mustard. 
After it splutters, add the fenugreek seeds and the remaining 2 red chillies and fry.
Add the small onions and fry for a few minutes.
Now add the tamarind extract and then the salt to taste, turmeric powder, asofetida powder and the curry leaves.( if adding other vegetables, add them now to the tamarind extract).
Allow to boil  until the raw smell goes and the vegetable is cooked well.
Add the mashed dal and the ground paste. Boil for a few more minutes.
If very watery, make a paste with 1/2 tsp of rice flour and water  and add to the sambhar to thicken it.  
Garnish with coriander leaves.

Instead of coconut, a little raw onion and tomatoes can be ground  into paste along with the other ingredients .
Serve hot with rice, idlis or dosas.