Put a large pan of salted water on to boil. Peel and finely slice the red onion. Pop it in a bowl and pour in enough boiling water to cover it. Set aside to soak.
Scrub the new potatoes and chop them into 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 finely slice the onion. Warm a frying pan for 1 min. Add 1 tbsp olive oil and the onion. Keep the heat low and fry for 8-10 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 onion fries, trim the French beans and slice into 2cm long chunks. When the potatoes have simmered for 12 mins, add the French beans to the pan. Simmer, without the lid on for 3 mins till the potatoes are cooked through and tender.
Drain the potatoes and French beans. Leave them to steam dry in the colander.
Tip the yogurt into a large bowl. Squeeze in the tahini. Add 1 tsp each of the turmeric and curry powder. Finely grate in the lemon zest. Squeeze in the juice from half the lemon. Add a good pinch and 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 them together so everything is well coated. Heap up into bowls and top with the crisp onions to serve.