Tasted this dish for the first time at Sahana and Vijay's place in Leeds. Loved it for its simple flavors. I like Beetroot for the unique color of the dishes and have cooked this dish many times after coming back from our UK holidays.
Here is Sahana's recipe for the dish

Method
1. Boil 1/2 cup of toor dal in pressure cooker. Along with Dal, you can also boil 2 beetroots by keeping them in separate vessels.
2. Grind 1/2 cup grated coconut with 2-3 tsp sambar powder with very little water to a fine paste. I used store bought sambar powder. I added 1 small piece of tamarind and some pieces of beetroot while making the paste.
3. Cut the boiled beetroot into pieces and keep aside.
4. Boil 1 glass of water in a vessel and while boiling add 2 green chillies and leave it for 5mins.
5. Add the beetroot pieces, boiled dal and of coconut and sambar powder and cook. Add 1 tsp of jaggery for some sweetness and a pinch of asafoetida and let this cook on a slow flame for at least 10 mins. You can adjust the water according to the consistency you desire.
6. For the tempering/tadka, take a small pan. Add 2 tsp coconut oil, add 1 tsp mustard seeds when the oil is hot. When the seeds start  to splutter, add 2 dry red chillies, 1 sprig of curry leaves and a pinch of urad dal. 
7. When the sambar is cooked, add the tempering/tadka and cover with the lid so that the flavor is absorbed.

Serve hot with rice.

Had an unused bunch of spring onion lying in my fridge for a week now..wanted to use it for my cooking today was in no mood for Chinese.Hence went online and googled for spring onion recipes and came across this nice Sambar recipe.
The sambar came very close to what is offered in udupi restaurants.

Method

1. Chop  around 20 stalks of spring onion along with the green leaves and keep aside. Finely chop 2 tomatoes and set aside.
2. Boil 1/2 cup of toor dal and keep aside.
3. Grind 1/2 cup of coconut, 2 tsp of cumin seeds, 2 tbsp of sambar powder,  pinch of turmeric powder, 1 marble sized ball of tamarind  and 1 tsp of jaggery powder to a fine paste with water and keep aside.
4. Heat 2 tsp of oil in a pan and add 2 tsp of mustard seeds. When they splutter add pinch of asafoetida and 1 sprig of curry leaves. 
5. Add the chopped Spring Onions &  tomatoes and mix well. Cook till the onions and tomatoes are soft.
6. Now add the ground paste, toor dal and 2 cups water and let the gravy boil for 5 mins. You can adjust the jaggery and salt now.
7. Garnish with coriander and serve with steamed rice.

I guess I can just replace the spring onions with onions, add more water and get a good sambar to go with hot idlis.