Put the potatoes in a large pot of salted water over medium-high heat, bring to a boil and reduce to a simmer. Cook until just fork-tender, 8 to 10 minutes. Drain and transfer to a baking sheet. Let rest for 10 minutes; this will allow the potatoes to steam and release excess moisture.
In a small bowl, mix the sea salt, pepper, smoked paprika, cayenne and onion powder.
Fill a large Dutch oven with 2 inches of oil and heat over medium heat to 375 degrees F. Carefully add half the potatoes and fry until golden brown and crisp, 4 to 5 minutes. Right before removing the potatoes, add a sprig of rosemary and half the sliced garlic to the oil and cook for 15 seconds. With a fine-mesh strainer, remove the batch to a medium bowl. Toss with half of the seasoning mix, then transfer to a paper towel-line baking sheet to drain. Repeat with the remaining potatoes, rosemary, garlic and seasoning mix.