Mix the mustard, clementine and lemon juice, zest, maple syrup, oil and some seasoning in a large bowl. Toast the pecans in a frying pan until a shade darker, then tip them into the bowl. Cut the beetroot, celeriac and apple into fine matchsticks (use a mandolin with a julienne attachment if you have one) and add to the bowl.
Add the onion, herbs and some seasoning, and toss until everything is well coated in the dressing. The slaw will keep for a few days in the fridge but the beet will stain all the other veg, so it will look best if served within 1-2 hrs.