First make the dressing. Put the ingredients (except the oil) into a small blender and whiz for a few seconds. Gradually add the oil and keep blitzing, on low setting, until you get a textured mixture. Taste it and season with salt and pepper to your liking.
Blanch the green beans in slightly salted, boiling water for 3-4 minutes, then drain. Alternatively, steam them over boiling water.
Cut the leeks into 6-8 cm pieces, then cook in lightly salted water for about 5 minutes and drain. Cut them in half (lengthwise), rub with a little oil, then grill, griddle or BBQ until lightly charred.
Put the vegetables in a serving bowl and mix with the dressing. Serve lukewarm.