Heat a medium 20cm oven-proof frying pan on medium high. When hot, drizzle with olive oil and add the onions. Cook, stirring often, until slightly softened, about 2 to 3 minutes.

Add the garlic, spinach, and season with 1 teaspoon of salt and lots of black pepper. Stir until the moisture has cooked out of the spinach and the mixture is mostly dry (a bit of moisture is fine), 8 to 10 minutes. Transfer the spinach to a bowl. Carefully rinse out the frying pan and dry it.

Preheat oven to 180˚C.

Using a sharp knife or mandoline, slice the potato as thinly as possible. Place ¼ cup of olive oil in a small bowl. Brush the bottom of the frying pan (or a similar sized baking dish, such as cake pan) with oil.

Working quickly so that the pastry does not dry out, lightly brush the top of one sheet with oil and then lay it in the pan, oiled side up, letting the edges hang over the edge. Continue brushing the filo sheets, and layering them into the pan, turning the pastry slightly each time, so there is an overhang of pastry all around (this is important for creating the border of the galette).

Scatter a third of the grated cheese across the bottom of the pastry, top with half of the spinach, spreading it out as evenly as possible. Next, layer half of the potatoes, overlapping them as needed to cover the spinach. Season lightly with salt and pepper. Add the remaining spinach, followed by another layer of potatoes, finishing with a layer of the remaining cheese.

Fold the pastry over, overlapping and tucking in the edges to create a border, leaving the centre exposed. It does not have to be uniform or overly neat. Brush the pastry with oil and scatter over the sesame or nigella seeds.

Place in the oven and cook until the pastry is deeply golden and the cheese is bubbling and golden on top, 40 to 45 minutes.

Allow to sit for 5 to 10 minutes. Cut into wedges and serve.