Heat 3 tbsp of oil in a ovenproof frying pan or cast iron skillet. Add leeks and cook over medium heat for 5 minutes. Then tip in cubed sweet potato and cook for 10 minutes covered with a lid. Make sure to stir once in a while.
Stir in kale, chopped garlic and chilli flakes, cover again and cook for a further 5 minutes. Then remove from the heat, season, stir gently to keep potatoes intact and set aside to cool down slightly.
In a separate large bowl, beat together your cream cheese, eggs, dill and lemon zest and some seasoning. Spoon the cooled down sweet potato filling into it and fold everything together.
Preheat your oven to 200C Fan. Wipe the pan you used for your vegetables. Lay the first sheet of filo pastry in it and brush with a little oil (no need to brush the pan first). Carry on layering the filo sheets, making sure the corners don't align (lay each sheet at a different angle) and brushing with a little oil in between.
Tip your creamy vegetable filling into your pastry covered pan and crumble in your goat's cheese on top (or arrange goat's cheese slices if you're using more mature cheese).
Crumple the pastry inwards over the filling, leaving the middle of the tart exposed and then brush the pastry with a little more oil. Sprinkle with nigella seeds.
Put your pan back on the hob on medium heat and cook for around 3 minutes to crisp up the bottom of your tart. Transfer to the oven and bake for 15 minutes until filo pastry is crispy and a lovely golden colour.
When you remove the pie from the oven, set it aside for 8-10 minutes. The filling will firm up, and it will be easier to remove from the pan and slice it.