Sift through red lentils and discard any stones, or misshaped or discoloured pieces. Rinse in a sieve under running cold water until the water runs clear.
Add lentils into a medium-sized saucepan with a fitted lid. Add stock and water, turn heat to high and bring to boil. Skim off any white foam that comes to the surface with a spoon and discard.
Turn heat down to a simmer, place lid on, and leave to cook for 20 minutes until a porridge-like consistency. Once finished cooking, remove from heat, with the lid left on.
Make the tomato-mix
While the lentils are cooking, peel and finely dice the onion and garlic. Grate ginger using the smallest hole on your grater or using a microplane grater.
Heat the oil or ghee in a fry pan over a low-medium heat. Once hot, add the cumin seeds and mustard seeds, and fry for a minute or so, until fragrant.
Add onions, garlic and ginger, and sauté for 15 minutes, or until onions are soft and clear.
Add turmeric and canned tomatoes, and cook for 10 minutes, stirring occasionally.
Combine and season
Once the spiced tomatoes have finished cooking, carefully add to the saucepan with the cooked lentils and mix.
Add spinach and chilli flakes, and mix. Bring to boil, then turn heat off and mix well.
Add the lemon juice and ½ - 1 tsp salt (to taste). Mix together, taste test and add more salt, lemon juice or chili flakes to your preferred seasoning.
Notes:
Serving suggestions: Great with rice or roti.* Sensitive to heat? You may prefer to knock this back to ¼ tsp chili flakes, or for a totally mild dahl add no chili flakes