- Brush a rectangular 20 cm/8 inch x 12 cm/5 inch baking dish with oil.
- In a medium bowl, whisk together the soured cream, skimmed milk, low-fat spread, cornstarch and pepper.
- Line the dish with one-third of the potato slices. Pour one-third of the soured cream mixture over the potatoes. Sprinkle half of the onion over the soured cream mixture. Repeat the layers in order: one-third of the potatoes, one-third of the soured cream mixture and the remaining onion.
- Arrange the remaining potatoes on the top and pour the remaining soured cream top mixture over the top. Cover with foil and bake in a preheated oven, 180*C (350*F), Gas Mark 4, for 1 hour. Remove the foil and bake for a further 20 minutes.
- Sprinkle with paprika and thyme sprigs, then let stand for 5 minutes before serving.