Rinse the lentils until the water is no longer murky, then drain and set aside.
In a large saucepan, heat the oil over a moderate heat and fry the onion for 5 minutes.
Reduce the heat and add the chopped garlic, ginger, finger chilli and whole mustard seeds and sauté for a couple of minutes. The mustard seeds should begin to make a popping sound.
Add a third of the tinned tomatoes and cook until slightly thickened. Next add all the spices, salt and pepper. Fry until the sauce forms a paste consistency, approximately 2 minutes.
Increase to a moderate heat and add the stock, lentils, coconut and the remaining tinned tomatoes bringing the dal to a low boil.
Reduce the heat, cover with the lid and simmer for 20 minutes.
Turn off the heat and stir through the lime juice. Taste and season with salt and pepper as needed.