Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Baking dish: spray a large baking dish with cooking oil and then set aside.
Sweet potatoes: use a vegetable peeler and chopping board to peel the sweet potatoes.
Potatoes: use a vegetable peeler and chopping board to peel the white potatoes.
Slice sweet potatoes: use a mandolin to slice the sweet potatoes into thin pieces.
Slice washed potatoes: next use the mandolin to slice the washed potatoes.
Cream sauce: add the cream, sour cream, garlic powder, onion powder, salt, pepper and chives to a medium bowl and whisk to combine. Note 3
Combine potatoes: add both the sliced washed potatoes and sliced sweet potatoes to a large mixing bowl. Use clean hands to toss to mix them together.
Into dish: spread some of the combined potatoes out into a layer in the baking dish. Then pour some of the sauce over the top and sprinkle a little cheese over the sauce. Repeat this process of layering the combined potatoes and sauce till you get to the top.
Oven bake: place the baking dish into the preheated oven on the middle shelf. Allow the potatoes to bake for 50 -60 minutes. Once the time is up, use oven mitts to remove the potato bake and place it onto a heatproof surface. Insert a skewer through the thickest parts of the dish. If there is any resistance, cook the potatoes for a little longer. Or top with both cheeses and then return to the oven for a further 5 - 10 minutes until the cheese is melted.
To serve: once the potatoes are cooked, use oven mitts to remove the baking dish from the oven and let the potatoes rest for 10 minutes. Then sprinkle chopped chives over the top and serve.