01 - Preheat the oven to 425°F. Line a large baking sheet with parchment paper.
02 - Place the potatoes in a large pot and cover with cold water. Add 1 tsp salt. Bring to a boil, then simmer for 15–20 minutes until fork-tender. Drain and let steam dry for 2 minutes.
03 - Arrange the potatoes on the prepared baking sheet. Use a potato masher or the bottom of a glass to gently flatten each potato to about ½ inch thick.
04 - In a small bowl, mix together melted butter, olive oil, minced garlic, sea salt, and pepper.
05 - Brush the garlic butter mixture generously over each smashed potato.
06 - Sprinkle the grated Parmesan evenly over the potatoes.
07 - Roast in the oven for 20–25 minutes, or until golden brown and crispy at the edges.
08 - Remove from the oven and sprinkle with chopped chives. Serve immediately.