Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and chop them into chunks around 1cm across and place them in a large ovenproof dish.
Peel and slice the red onion into around 8 wedges. Halve the courgettes and slice them into 1-2cm thick chunks. Add the veg to the dish.
Drizzle over 1/2 tbsp olive oil and season with a pinch of salt and pepper. Toss the veg to mix. Slide into the oven and roast for 25 mins.
While the veg roasts, tip half the yogurt into a dish. Add 2 tsp mild curry powder and a pinch of salt. Squeeze in the juice from half the lemon. Peel and grate or crush the garlic, then add that too. Stir well to mix.
Add the hake to the mix and turn to coat completely in the tandoori marinade. Let it marinate for at least 10 mins.
Halve the cherry tomatoes. Take the dish out of the oven and add the cherry tomatoes. Stir to mix. Lift the hake fillets out of the dish and nestle them in amongst the veg. Roast for 10-15 mins, till the veg are tender and a little charred and the salmon is cooked through. When you press a fork into the hake, it should flake easily.
Arrange the roast hake and veg on 2 warm plates and serve with the remaining yogurt on the side and lemon wedges for squeezing.