Preheat the oven to 200°C / fan 180°C / gas mark 6. Toss the sweet potato in 1 tbsp oil, paprika and fresh ground black pepper and pop onto a baking tray. Put into the oven and bake for 15 minutes.
Meanwhile, place the cauliflower core side down and using a large sharp knife slice the cauliflower from top to base into four “steaks”, about ¾ of an inch thick. Season each steak with salt and pepper on both sides, reserving any loose florets for use in a different recipe.
Heat 1 tbsp oil in a large frying pan over a medium-high heat. Add the cauliflower and sear until golden brown, roughly 2 minutes on each side. Transfer the steaks to a baking sheet.
Take out the sweet potato fries and give them a quick toss, then move them to the top shelf, putting the cauliflower steaks on the middle shelf, baking both for 20 minutes.
Meanwhile, for the chimmichurri, pulse the garlic cloves, parsley and coriander until finely chopped, then add the rest of the olive oil, vinegar, chilli, and juice and zest of the lemon and give a final pulse.
When ready to serve, place the cauliflower steaks on a plate and spoon over the chimmichurri sauce, with sweet potato fries on the side.