Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Potatoes: use a vegetable peeler and cutting board to peel the potatoes. Then discard the skins. Once done, use a mandolin or knife and thinly slice the potatoes. Then use a knife to cut the potato slices in halves (optional).
Cream sauce: next pour the cream into a large bowl and add the mustard powder, garlic powder, dried thyme, salt and pepper. Stir to consistently combine these and then set aside.
Combine: add the potato sliced, cubes of ham and diced onion to a large mixing bowl. Then use a wooden spoon to stir and combine. Once combined pour the cream sauce over the top and stir again until it is mixed through.
Baking dish: transfer the combined ham potato and onion mixture into a greased baking dish. Gently pat the surface to ensure it is a flat layer with no potato slices sticking up too much. Then place the baking tray into the oven (middle shelf).
Oven bake and cheese topping: let the potato bake cook for 50 minutes. Once the cook time is up, use oven mitts to remove the baking dish and place it onto a heatproof surface. Then sprinkle the colby cheese followed by the mozzarella cheese over the top. Return the baking dish to the oven and let the cheese melt for around 10 minutes. Then turn the oven off and let the potatoes sit in there for 5 minutes.
To serve: use oven mitts to remove the baking dish from the oven and place it onto a heatproof surface. Allow it to start to cool and the cream sauce stop bubbling and thicken. Once it has, place the potato and ham bake onto a heatproof surface on the table and garnish with chopped parsley.