Peel and halve (or cut in three for larger potatoes).
Parboil 10 minutes at most.
Preheat your oven (about 180C) and your baking dish so the butter and veg oil will melt together.
When the potatoes are ready at parboil stage, drain, and shake your pot so the potatoes get all scuffed up on the outside.
Place in your dish (don't forget the bits that were around the edge of the pot) and turn about so that they are coated with the melted butter and oil. Put them in the preheated oven (the dish should already be hot).
After about half an hour, check on your potatoes and turn them in the dish - they tend to brown on the bottom, and it's an all-over effect that is extra delicious.
Roast for another half hour, getting the rest of your dinner ready in the meantime. These are so delicious - perfectly crispy on the outside and melt-in-your-mouth fluffy on the inside. One of the reasons I think it took so long to make such delicious potatoes is that I love other roasted veggies and would put them all in the same dish. The moisture from the other vegetables I think kept the potatoes from browning and crisping, so I cook them separately now!