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Poke the purple potatoes all over with a fork and transfer to a large pot filled with water.
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Bring the water to a boil and let cook for 10-15 minutes (depending on the size of your potatoes) or until you can pierce a fork through them.
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Remove from heat and drain.
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Preheat oven to 400 degrees and line a baking sheet with parchment paper.
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Spread 2 tablespoons of olive oil along the parchment paper and let warm up in the oven for 5 minutes or until hot.
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Add the potatoes, pepper (if using), and onion (if using) to the parchment paper.
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Use a large fork or back of a spoon to smash the potatoes.
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Whisk together the rest of the olive oil, butter, rosemary and garlic and then spread over the vegetables. Season with salt and pepper.
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Cook for 15 minutes, flip the vegetables, and cook for another 10 or until everything is crispy.
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Remove from oven and serve warm.