Preheat oven to 200˚C / 400˚F. Finely slice the stalks and the leaves of the greens and place in separate bowls.
Heat a large skillet (frying pan) on medium high heat. Add 1-2 tablespoons of olive oil, along with the leeks. Toss until softened, about 2-3 minutes. Add the greens stems and toss until softened, about 1 minute. Then add the leaves and the garlic and season with 1/2 - 1 teaspoon of salt, a big pinch of black pepper and cook for 2-3 minutes until the leaves are wilted and most of the liquid has cooked off (some moisture at the bottom of the pan is fine).
Place the cheese into a bowl and add 1/2 of the herbs. Toss gently to combine.
Line a 23cm / 9-inch round pan (I use a springform) with baking paper (no need to spray with non-stick oil). Drizzle with oil and tilt it to coat the base. Add a layer of potatoes, overlapping them until the base is covered. Sprinkle with sea salt and black pepper. Top with half the greens, spreading it out into an even layer and then top with 1/3 of the herby cheese, and drizzle with olive oil.
Add another layer of overlapping potatoes, then season with salt, black pepper, and then top with the remaining greens, half of the remaining cheese and drizzle with oil.
This is the final layer. Add the remaining potatoes in an overlapping layer (use up all your potatoes, even if you need to do a double layer), season with salt and black pepper and top with the remaining cheese. Drizzle with olive oil and cover tightly with foil.
If using a springform pan, put it onto a baking sheet to catch drips. Bake for 40 minutes. Remove the pan from the oven, and increase heat to 230˚c / 450˚f. Remove the foil and place the pan back into the oven, baking until the top is golden, about 15-20 minutes.
Remove from the oven and allow to cool for 10-15 minutes. Top with remaining herbs, slice into wedges and serve as is or with a side of salad.