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Bring a frying pan of salted boiling water to the boil, reduce to a simmer, add the leeks and cover with a cartouche (a piece of baking parchment with a hole in the middle). Cook for 4-5 mins until tender and a deeper green, then remove from the water with a slotted spoon (keeping the water at a simmer) and dry in a clean tea towel.
Add the runner beans to the water, cook for 2-3 mins until tender, then drain and set aside.
Heat a griddle pan over a high heat and toss the leeks in 1/ 2 tbsp olive oil and some seasoning. Griddle for a couple of mins each side until charred. Transfer to a chopping board and slice into lengths the same size as the runner beans.
To make the vinaigrette, pound the anchovy using a pestle and mortar (or whizz in a blender), add the chilli flakes, Dijon, red wine vinegar and sugar and combine. Whisk in the remaining olive oil, season, then stir in the herbs.
Put the runner beans and leeks in a large bowl. Pour over the vinaigrette, toss in the spring onions, then scatter over the seeds.