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If your oven does not have a Preheat feature that alerts you when it reaches temperature, check to see if it has a thermometer inside that will indicate when it's hot enough. Otherwise, put the potatoes in whenever they're ready -- they may just take a little longer to roast. When the potatoes are done, you'll be able to push a fork into one without any resistance. The skins should also be slightly wrinkled and golden, and you should be able to smell them strongly. You can coarsely chop some fresh parsley or mint and add it to the potatoes after they cool for some extra flavor. You may also want to add a little extra olive oil if the potatoes seem dry.
Preheat the oven to 400F (204C). Bake the potatoes for another 20 minutes or until they're soft. Put the potatoes in a serving dish and garnish them.