Put the lentils in a saucepan, cover with cold water and add a pinch of salt. Bring to the boil and cook for 20—25 minutes until just cooked but not mushy. Drain and refresh in cold water, then drain again and transfer to a large salad bowl.
Add the carrots and celery to the bowl with the lentils. Crumble in the feta and add the chopped parsley.
Make the dressing by whisking the vinegar, mustard and oil. Add the dressing to the salad and stir to combine well. Season to taste with salt and pepper and serve immediately.