Tip the rice into a bowl and cover with cold water. Set aside to soak.
Peel and finely chop the onion. Peel and grate the ginger and garlic. Finely chop the chilli (for less heat, flick out the seeds). Chop the lemon grass stem into four pieces. Bash with a rolling pin or pestle a few times to bruise it.
Drain the rice and tip it into a small pan. Pour in 150ml boiling water. Add a pinch of salt. Cover. Bring to the boil. Turn the heat right down and gently cook for 25 mins, then take off the heat to steam in the pan, lid on, for 5 mins.
Warm 2 tsp olive oil in a small pan. Add the onion. Season with a pinch of salt and pepper. Cook for 5 mins, stirring now and then, till the onion has softened. While the onion cooks, roughly chop the tomatoes.
Stir the garlic, ginger, chilli and lemon grass into the onion. Add the Bombay spice mix, 1 tsp black mustard seeds and the tamarind paste. Scoop in the tomatoes. Cover. Simmer for 20 mins, stirring occasionally, till the tomatoes have broken down.
While the curry sauce and rice cook, coarsely grate the courgette. Chop the hake fillets into bite-size chunks.
The rice should be tender by now. Stir in the courgette. Put the lid back on the pan and let it steam for 5 mins.
Add the hake to the curry. Cover. Simmer for 5 mins till the fish is opaque and flaking. Taste the sauce and add salt and pepper if you think it needs it. Fish out the lemon grass pieces and discard them. Spoon the courgette rice into two warm bowls. Ladle over the curry and serve.