Heat your oven to 190°C/Fan 170°C/Gas 5 and put a roasting tin on the top shelf to warm up. Scrub the new potatoes and chop them into chunks around 2-3cm across. Pop them in a large bowl.
Add 1 tsp each cumin seeds and coriander seeds to a mortar and pestle and roughly crush them. (No mortar and pestle? Use a small bowl and a jar or the end of a rolling pin to crush the spices.) Add the spices to the potatoes with a good pinch of salt and pepper and 1 tbsp olive oil. Turn the potatoes to coat. Take the roasting tin out of the oven and add the potatoes. Slide back into the oven and roast the spuds for 20 mins.
While the potatoes roast, peel the onion and slice a sliver off one side so the onion sits flat on your chopping board. Slice it into thin rings. Add the onion to the bowl that had the oil and spices in and turn to pick up any last bits of oil and seasoning. Set aside.
Spoon the yogurt into a separate bowl. Trim and finely chop the white part of the spring onion. Add to the bowl. Finely grate in the lemon zest (keep the lemon for later). Finely chop the chives and add them to the bowl with a pinch of salt and pepper. Stir together.
When the potatoes have roasted for 20 mins, take them out of the oven and add the onion rings to the dish. Toss to mix together, then return to the oven on the top shelf. Roast for another 10 mins.
Line a baking tray with baking paper. Lay the salmon fillets on the baking tray, skin-side down. Spoon 3 tbsp of the yogurt mix over each salmon fillet. Pop the rest in the fridge for later.
Slide the salmon fillets into the oven on the shelf below the new potatoes and bake for 10 mins. This means the potatoes will roast for 40 mins in total. The potatoes are ready when they're golden and crisp. The salmon will have a pale brown crust and will flake easily if pressed with a fork.
While the salmon roasts, squeeze the juice from half them lemon into a bowl and whisk in 1 tbsp oil and some salt and pepper. Toss the peppery salad leaves with the lemon dressing.
When the salmon is ready, pop it onto 2 warm plates and serve with the potatoes, the rest of the lemon and chive yogurt and the salad leaves.