Tip the lentils into a large bowl and cover with cold water. Set aside to soak for a few mins.
While the lentils soak, peel and finely chop the onion. Halve the green chilli and finely chop it, flicking out the seeds and white bits for less heat. Peel and grate 1 garlic clove. Finely chop the stalks from the coriander, keeping the leaves for later.
Warm a medium pan for 1 min, then add 1 tsp olive oil and the onion. Season with salt and pepper and cook over a low heat for 5 mins, stirring occasionally till golden and soft. If the onion starts to stick, add a splash of water. Drain the lentils.
Add 1 tsp of the turmeric, the chilli, garlic and coriander stems to the onion. Add 1 tsp each of the cumin seeds and curry powder (keep the rest for later). Add 2 tsp each curry leaves and mustard seeds. Crumble in the stock cube. Give everything a good stir.
Stir in the lentils. Pour in 600ml boiling water. Shake the tin of coconut milk a few times then pour that in too. Cover the pan. Turn the heat up and bring to the boil.
When the daal is boiling turn the heat down to medium-low and simmer for 30 mins till thick. Stir now and then to stop it sticking. You should end up with a soft, soupy daal so if it looks too dry or stiff, add a splash more water.
While the daal is cooking, warm a frying pan for 1 min. Add 2 tsp olive oil and the beef mince. Stir and fry over a medium heat for 12-15 mins till the beef is well browned and cooked through.
While the beef fries, peel and finely grate the remaining garlic clove and the ginger.
Add the garlic, ginger, remaining cumin seeds and curry powder to the beef mince with a pinch of salt and pepper. Cook and stir for 2 more mins. Add the spinach to the lentils and fork through to wilt it. Taste and add more salt and pepper if it needs it. Ladle the daal into warm bowls and top with the beef and coriander leaves to serve.