Preheat oven to 200°C. Spray a 6cm deep, 15cm x 24cm ovenproof dish.
Combine the yoghurt, milk and mustard in a small mixing bowl. Spoon 1/3 of the yoghurt mixture over the bottom of the baking dish.
Arrange half the potatoes over the yoghurt. Top with a layer of onion, 1.5 cups spinach leaves and 1 tablespoon thyme leaves.
Repeat by layering another 1/3 of the yoghurt mixture, followed by a layer of potatoes, 1 1/2 cups spinach leaves and 1 tablespoon thyme leaves.
Finish with a layer of the remaining yoghurt mixture and sprinkle cheese evenly over the top.
Cover baking dish tightly with foil. Bake for approximately 1 hour or until potatoes are just tender. Cook for an additional 30 minutes to 1 hour until potatoes are soft. Place under grill until top is golden.