Place potatoes in a large pot and cover with cold water salted with 1 tablespoon of salt.
Bring to a boil over high heat, then reduce to a simmer and cook for 15 to 20 minutes until fork-tender.
Drain the potatoes and allow them to sit in the colander for 5 minutes to ensure surface moisture evaporates.
Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius) and line a large rimmed baking sheet with parchment paper.
Transfer potatoes to the baking sheet and use the bottom of a heavy glass or a potato masher to press each potato down to a thickness of approximately 1/2 inch.
Drizzle the smashed potatoes with olive oil and sprinkle evenly with salt, pepper, garlic powder, and rosemary.
Roast in the preheated oven for 25 to 30 minutes, turning once halfway through, until the edges are deep golden brown and crispy.
Serve immediately while hot for optimal texture.