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 spray then set aside.
Potatoes: use a vegetable peeler and cutting board to peel the potatoes and discard the skins. Then use a mandolin to thinly slice the potatoes.
Cauliflower: use a knife and cutting board to remove the stem from the base of the cauliflower. Then chop the whole head into florets and small chunky pieces.
Cream sauce: pour the cream into a large mixing bowl. Then add the mustard powder, dried chives, garlic powder, salt and pepper. Stir till consistently combined then set aside.
Vegetables: add the sliced potato, cauliflower pieces and diced onion to a large mixing bowl. Then use a wooden spoon to stir till consistently mixed.
Combine: pour the cream sauce over the combined vegetables and stir until it is consistently combined.
Oven bake: transfer the combined potato, cauliflower, onion and sauce mixture into the greased baking dish. Gently push the surface pieces down slightly so that they are in a reasonably flat layer. Then place the baking dish into the preheated oven on the middle shelf and let it bake for 50 mins. Once the cooking time is up, use oven mitts to remove the cauliflower bake and place it onto a heatproof surface. Use a skewer and insert it through a thick part of the potato bake. Ensure it slides straight through easily. If it doesn't, return the dish to the oven until a retest achieves this.
Cheese topping: sprinkle the mozzarella cheese over the cooked potato cauliflower bake followed by the grated colby cheese. Then place the baking dish back into the oven for 10 minutes (same temperature as earlier). Let the cheese melt and the surface of the potato bake turn a light golden brown. Then use the oven mitts again to remove the baking dish and place it onto a heatproof surface.
To serve: allow the potato bake to cool for around 10 minutes (the sauce will stop bubbling and thicken during this time). Then garnish with chopped parsley and place in the centre of the table with a large spoon to serve.