Arrange the nasturtium leaves around the outside of a large flat plate. Lay the flowers on top of the leaves with their stem ends pointing toward the centre of the dish. Working towards the centre of the plate, ad a layer of onions, then a layer of tomatoes. Continue layering until the dish is full. Cover the salad & refrigerate for at least 2 hours to let the flavours blend. Sprinkle with celery & drizzle with the vinaigrette. Serve immediately.