Coarsely grate half the cucumber and pop into a sieve set over a bowl. Add a pinch of salt and scrunch the cucumber with your hands for 2-3 mins to squeeze out as much liquid as you can. Set aside to drain.
Peel and finely dice the onion. Peel and finely dice the garlic and ginger. Halve the chilli. Flick out the white bits and seeds for less heat. Finely chop it.
Heat 1 tbsp oil in a large frying pan. Add the lamb mince. Cook over a high heat for 2 mins. Stir with a wooden spoon to break up the meat. The meat should start to brown. Drain off any excess liquid. Add the onion to the pan. Stir and fry for 5 mins till the onion looks soft and glossy and the mince has browned a bit more. Season with salt and pepper.
Turn the heat down and stir in the garlic, ginger and chilli. Cook and stir for 1 min till the pan smells aromatic. Measure out 2 tsp garam masala and add to the pan with 1 tsp ground cumin and ½ tsp turmeric. Cook and stir for 2 mins till fragrant.
Add the tomatoes to the pan. Add 300ml boiling water and give a good stir. Cover. Simmer for 15 mins till the sauce has thickened and the mince is cooked.
While the lamb keema cooks, tip the rice into a small pan. Pour in 300ml boiling water. Cover. Bring to the boil. Turn the heat right down. Simmer for 6-8 mins till all the water is absorbed. Take off the heat. Set aside, lid on, for 5 mins.
Squeeze the cucumber to wring out any excess moisture and transfer the cucumber to a bowl. Stir in the yogurt with a grinding of black pepper and ½ tsp ground cumin. This is your raita. Set aside.
Stir the spinach into the lamb keema. Simmer, uncovered, for 2-3 mins to just wilt the spinach. Taste the keema and season if you think it needs it. Serve with the rice and cucumber raita, garnished with the coriander leaves.