If you travel in South India, you'll probably eat rice and sambar a lot. This combination, along with chutneys, pickles and maybe some stewed vegetables with coconut, is the staple diet of the majority of people in the south and is the equivalent of the chappati-based thalis served further north. In smaller towns there are often only one or two basic restaurants, usually advertising 'ready meals'. This generally means a plate of rice and sambar plus a few bits and pieces, which can range in quality from dire to delicious. At the delicious end of the scale, a ready meal can be an unforgettable culinary experience. In fact, on reflection, I would say that at the other end of the scale the same also applies.
Let's be clear and state that here, on this page, we're dealing with the delicious end of the scale. Since rice is such a central ingredient in a south Indian thali, I recommend using good rice. Basmati is my favourite and is widely available, although it's more a north Indian speciality. There are excellent varieties of rice in south India too, especially in Kerela, but I would think they are difficult to obtain elsewhere.
There is a saying in English, 'You can't make a silk purse from a sow's ear'. I would rather not try, as I imagine the sow would prefer to keep it's ear, rather than have me experimenting with it. I prefer the Russian saying 'You can't make sweets from shit', and I think this is quite appropriately applied to sambar. One potato, a handful of lentils and five litres of water will not produce good sambar. I've given a list of ingredients below for a basic, but tasty sauce. Feel free to experiment with other vegetables, for example cabage, cauliflower, green peas or peppers and, of course, if you want to make authentic south Indian sambar, throw in about twenty green chillies. I have another sambar recipe too, if you care to try that version.
Sambar is usually very thin, but the amount of water I recommend will make a thicker version. Depending on the consistency of the other dishes you choose to serve, you might want to go for the watery version, in which case it's easy to thin the sauce at the end of cooking time with hot water straight from the kettle.
Heat some oil in a pan and when it's hot, throw in the mustard seeds and a few seconds later, the chopped onion, chopped ginger and pounded cumin and coriander seeds. When the onion becomes transparent, add chopped potato, carrot, tomatoes and garlic. Fry for a minute or two, then add the lentils, turmeric and water. Bring to the boil then simmer until the lentils are soft and breaking up. Add salt lemon and methi near the end of cooking time.