Preheat oven to 200°C. Lightly grease a large, deep baking dish.
Heat oil in a frying pan over medium heat. Add kale and cook until softened. Remove from pan and set aside.
Add leek to pan and cook, stirring, for about 5 minutes or until soft. Remove from pan and set aside.
Slice potatoes on a mandolin.
In a saucepan over medium heat, bring together cream, Italian seasoning, olive paste, butter and herb salt.
To build gratin, first place one-third of the potato slices in prepared baking dish. Follow with half of the kale, then half of the leek, then one-third of the cheese.
Next up, add another one-third of the potato followed by the remaining kale and leek, and another one-third of the cheese.
Finally, add remaining potato then pour over the cream. Top with remaining cheese then cover dish and bake in oven for 35–40 minutes, until cooked through and golden-brown on top.