If you are making your own pastry place all the ingredients in a food processor and blitz until you have a smooth dough. Alternatively rub the butter into the other ingredients until a dough forms. Cover and chill for 20 mins in the fridge.
Preheat the oven to 200C. Roll out the chilled pastry (bought or homemade) on a lightly floured surface until a couple of millimetres thick.
Use it to line a fluted 23cm tart tin (loose bottomed works best) trimming off any excess. Cover with greaseproof and baking beans and blind bake for 10 minutes. Remove the greaseproof and return to the oven for a further five minutes.
Set aside whilst you prepare the filling. Peel and roughly chop the sweet potato. Scatter over a baking tray and drizzle with olive oil and light seasoning. Roast in the oven for 20 minutes or until golden brown and soft.
In the mean time, finely slice the shallots and sweat in a saucepan to soften, then set aside to cool.
Chop the cavelo nero roughly. In a large bowl whisk together the eggs and cream. Stir in the green and the cooled shallots. Finely slice the sage and add to the bowl alongside a touch of nutmeg. Whisk together with the fresh thyme and parmesan.
To assemble, spoon the roasted sweet potato into the tart then pour over the egg mixture. Grate a little more parmesan on top then place in the oven. Immediately turn the oven down to 180C. Bake for 25 minutes or until well risen and golden brown on top. Allow to cool before serving.