Tip the rice into a large bowl and cover with cold water. Whisk for 1-2 mins till the water goes cloudy. Drain and rinse under cold water.
Tip the rice into a small pan. Add 300ml boiling water and a pinch of salt. Cover. Bring to the boil. Turn the heat right down and gently simmer the rice for 25 mins till it’s tender and the water is absorbed. Take off the heat. Set aside, lid on, for 5 mins to finish cooking the rice.
While the rice simmers, peel and chop the onion. Peel and finely slice the garlic. Peel and grate the turmeric. Trim and chop the rhubarb into 2cm pieces. Heat a deep frying pan or wok and add 1 tbsp oil. Add the onion and garlic. Stir fry for 5 mins.
Add 2 tsp garam masala to the onions and cook for 2 mins. Add the turmeric. Tip in the red lentils and stir thoroughly to mix with the spiced onions.
Pour in 150ml boiling water and stir for 3 mins till absorbed. Add the rhubarb with 150ml boiling water. Add 1 tbsp Demerara sugar. Stir and cook for 10 mins till the rhubarb is just soft and the lentils are cooked through.
While the lentils simmer, grate or pare the zest from the lemon. Stir into the yogurt. Juice 1 half and slice the other half into wedges to serve. Pour the juice into the lentils. Stir the spinach through the lentils to just wilt it.
Roughly chop the coriander leaves. Stir half of the chopped coriander through the lentils, saving the rest for later. Fluff up the rice with a fork.
Serve the lentil and rhubarb curry with the rice. Top with a dollop of zingy yogurt and a scattering of the remaining coriander leaves. Pop a lemon wedge on the side for squeezing.