Pour the clementine juice into a small saucepan and place it over a medium heat. Bring the juice to a boil and reduce it until has thickened enough to easily coat the back of a spoon (about 3–4 minutes). Add the butter and grated zest, reduce the heat to melt the butter but do not allow the liquid to boil or the acid in the juice might split the sauce.
Pour the butter sauce into an ice-cube tray, a bowl or a small lidded food box, leave it to cool, then chill it until solid.
Boil the carrots and parsnips in salted water until al dente, then drain.
Place a frying pan over a medium heat. Sear the clementine halves to serve, cut side downward, until caramelized. Remove from the pan and set aside, then add the vegetables to the dry pan and sear until golden.
Add the honey, mustard and thyme and stir to glaze and lightly caramelize the veggies. Add a good knob of clementine butter and a little splash of water – the glaze will start to bubble and will coat the carrots and parsnips in a beautiful, shiny way. Season with salt and pepper, remove the thyme sprig, then scatter with the parsley leaves and extra clementine zest, tuck in the seared clementine halves, and serve.