Put a large, covered pan of salted water on to boil. Peel and thinly slice the red onion and pop it in a heatproof bowl. Pour in enough boiling water to cover it. Set aside to soak.
Scrub the potatoes and chop them into small equal-sized chunks, around 2–3cm across. When the water is boiling, add the potatoes and simmer for 12 mins.
While the potatoes cook, peel and thinly slice the shallots. Warm a frying pan for 1 min. Add 1 tbsp olive oil and the shallots. Keep the heat low and fry for 5–8 mins till golden and crisp. Lift out of the pan with a slotted spoon and pop on a plate lined with kitchen paper to drain.
While the shallots fry, trim the woody ends off the French beans. When the potatoes have simmered for 12 mins, add the beans to the pan. Simmer, without the lid on, for 3 mins till the potatoes are cooked through and the beans are just tender Drain the potatoes and French beans. Leave them to steam dry in the colander.
Tip the yogurt into a large bowl. Add 1 tsp each turmeric and curry powder. Finely grate in the lemon zest. Squeeze in the juice from half the lemon. Add a good pinch of salt and pepper and stir together to make the dressing.
Tip the potatoes and beans into the bowl with the yogurt dressing. Drain the red onions and add them too. Toss together so everything is well coated. Heap up into bowls and top with the crisp shallots to serve.