Prepare the potato: Wash the potatoes without peeling the skin. Slice in half.
Parboil: Add the potatoes and turnips, salt, and water to the pot and boil for 10 minutes on medium-low heat. Immediately drain the water, and let the potatoes cool until it no longer releases moisture. This step will ensure that the potatoes cook fast. There is no need to remove them from the oven and flip onto the other side.
Roast the potatoes: Preheat the oven to 220 °C/450 °F. Transfer the potatoes from the pot to a casserole/baking tray, coat them with sunflower oil and butter, and roast them for 15 minutes and then lower the temperature to 145 °C for the duration of the baking time.
Notes
Storage: Store in an airtight container in the fridge for two to three days.