Trim and finely shred the chard or kale. Toast the walnuts in a dry pan lightly for a couple of minutes. Take the walnuts out of the pan and heat a little olive oil. Cook the chard with the red onion until slightly softened. Allow to cool slightly before putting into a bowl with the celery. Mix the dressing ingredients together and toss through the vegetables. Quarter the apples and slice as thinly as you can before adding this to the salad too. Add the lentils and walnuts. Season, toss and serve.