Tip the rice into a bowl and cover with cold water. Set aside while you prepare the veg. Peel and finely chop the onion. Halve the cherry tomatoes. Finely grate the zest from the lemon.
Drain the rice and tip it into a small pan. Pour in 300ml boiling water and add a pinch of salt. Cover, bring to the boil and then turn the heat right down. Gently simmer for 8 mins till all the water has been absorbed. Take off the heat and leave in the pan, lid on, for 5-10 mins to steam the rice. It will keep warm in the pan while you finish cooking the rest of your meal.
While the rice simmers, warm a deep frying pan or wok over a medium heat for 2 mins, then add the onion with ½ tbsp oil. Season with a pinch of salt and pepper and fry for 5 mins till the onion has started to soften.
Stir in the tomatoes, lemon zest and Goan curry paste. Cook and stir for 2 mins, then pour in the coconut milk. Pop a lid on the pan, turn the heat down a little, and gently simmer for 15 mins, stirring now and then, till the tomatoes have started to collapse and you have a thick, fragrant sauce. Taste and add a pinch more salt or pepper if you think it needs it.
Stir the spinach into the sauce and leave it to simmer for 2 more mins to wilt the spinach.
Slide the plaice fillets into the sauce. Simmer for 4-5 mins till the plaice fillets are cooked – they'll flake easily when pressed with a fork.
Fluff the rice and heap up on 2 warm plates. Top with the plaice fillets and the coconut sauce and serve, garnished with coriander leaves.