Preheat the oven to 200 C / Gas 6. Butter a casserole dish to prevent sticking. Fry Light® works a treat!
Peel and slice your potatoes to about 1/2 cm slices. I would suggest cutting the slices in half, especially if using large potatoes, to ensure the potatoes cook nice and soft.
Layer the potatoes in the bottom of your casserole dish.
Begin to make the cheese sauce: Melt butter in a pot, then add flour. Mix thoroughly until you have a nice paste. Add milk and stir constantly with a whisk until the sauce just boils. It should begin to thicken at this stage.
When sauce has thickened, remove from heat and immediately add the cheese. Mix until cheese has completely melted. Then stir in the crumbled stock cubes and the diced onion. Mix until stock cubes have completely dissolved. Pour sauce evenly over potatoes. Cover your dish.
Bake for about 1 1/2 hours. When finished, your potatoes should be soft and the cheese sauce should just begin to brown and crisp around the edges.