*Instead of tomatoes, I use tamarind (size of 1 lime, soaked in water). If you are gravy lover, add more water before adding chopped tomatoes. This lentil curry is my own version and hope you'd enjoy it as everyone else. We Sri Lankans are very fond of lentil curry and we have this curry atleast a couple of times a week. Tamarind Pod is a tropical fruit that we sun-dry when they are ripe. Tamarind pod has a semi-hard [easily crackable] peel, and when you break the peel, you can see "sticky type" of pulp which has covered its seeds [like beads]. You can buy tamarind pulp in Asian Food Stores, and all you need to do is just break a small piece and soak in a little bit of water and squeeze the juice and use in the curry. I live in Cairns, Australia - and I recently found "tamarind pulp" [comes in 4"x4"x1" block, wrapped in clear plastic wrapping, which you can keep forever in an air-tight container and use whenever you need] in an Asian Food Store.
Tamarind has a sour taste together with a bit of sweetness - but, you can easily substitute it with chopped well ripen tomatoes. Hope these information would be helpful to you.