Heat half the oil in a saucepan. Add the spinach and stir. Cover and cook for 10-15 minutes, stirring occasionally. Fry the mushrooms separately with the remainder of the oil. Drain the spinach (keep the water for a healthy, vitamin-enriched juice or stock) and mix with salt, pepper, fried mushrooms and cottage cheese.
Sift the flour and salt into a bowl. Rub in the margarine and add water to make a soft dough. Roll the dough out to 1cm (½ in) thick on a floured board. Grease a 25x30cm (10x12 in) oven-proof dish. Place the rolled pastry in the dish and prick the pastry base with a fork before baking. Bake on the middle shelf at 190°C/375°F/Gas Mark 5 for 5-8 minutes or until golden brown.
Once pastry is cooked, spread the spinach mixture evenly on top. Sprinkle with grated cheese and sesame seeds. Place back in the oven for a few minutes until the cheese has melted. Serve with baked potatoes and salad.