Position a rack in the lowest third of the oven. Preheat the oven to 450 degrees. Line the bottom of a large, shallow-sided baking sheet with parchment paper.
In a medium bowl, toss the potatoes with the olive oil, rosemary and 1-1/2 teaspoons of salt; spread in a single layer on the baking sheet. Roast for 30 minutes, rotating the baking sheet once after 15 minutes.
Remove the potatoes from the oven, flip them over and roast until golden, about 5 more minutes. Sprinkle with salt.
Meanwhile, in a small bowl, stir together the mayonnaise, lemon juice and lemon zest. Season the dressing to taste with salt and pepper. Serve the lemon mayo alongside the potatoes.