Cook the potato slices in a large saucepan of salted boiling water for 5 minutes, then drain well.
Meanwhile, cook the spinach in a separate saucepan of boiling water for 1–2 minutes. Drain and squeeze out the excess water.
Grease a large ovenproof dish and line the bottom with half the potato slices. Cover with the spinach and half the mozzarella, seasoning each layer well with salt and pepper. Cover with the remaining potato slices and arrange the tomato slices on top. Sprinkle with the remaining mozzarella.
Whisk the eggs and cream together in a bowl and season well with salt and pepper. Pour over the ingredients in the dish.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 30 minutes. Serve immediately with a salad and crusty bread.