Wash the red lentils in a large pan at least 3 times in warm water until the water comes out clear. Add 1.5L of fresh water into the pan and boil for 15 minutes.
Blend the chopped tomatoes in a hand blender. Bash 4 cm of ginger, 4 cloves garlic and 2 teaspoons of chilli powder in a mortar and pestle to form a thick paste.
In a large pan, pour two glugs of vegetable oil. Once the oil is hot, add the black mustard seeds and cumin seeds. Wait for them to start dancing around in the pan, then carefully add in the chopped tomatoes. Quickly pop a lid on - they will spit out so be careful.
Add in the turmeric, cumin powder, coriander powder, 2 teaspoons of chilli powder, 2 teaspoons of salt and the pounded garlic and ginger paste. Cook on a medium heat for at least 10 minutes, stirring frequently.
For the garlic oil and toppings, thinly slice 6 cloves of garlic and the green chilli, finely chop the coriander and chop 4cm ginger into matchsticks.
To check if the dhal is cooked, press a few grains between your fingers to feel if they are tender. Drain the dhal and add it into the curried tomatoes. Add in 250ml of water and continue cooking for 2 minutes. Season to taste and stir in 3/4 of the coriander.
Heat up 50ml of vegetable oil in a small pan and gently cook the garlic and curry leaves until golden. Pour over the dhal and put a lid on to allow the garlic flavour to infuse through.
Finish with the remaining ginger, chilli and coriander and serve with a big bowl of rice or just on its own.