Preheat the oven to 425° on convection heat or 450° on regular bake.
Scrub the potatoes with a vegetable brush under cold water. Cut the potatoes into approximately ¾-inch cubes, place in a bowl, and cover with cold water to prevent the potatoes from turning brown.
Drain the potatoes and pat them with a cloth towel to dry them as much as possible. Place the potatoes in a baking pan large enough to accommodate them snugly in a single layer (some overlap is fine). Lightly crush and peel the garlic cloves and add them to the potatoes with the rosemary and olive oil. Season generously with salt and pepper. Stir to coat the potatoes with the olive oil.
Bake until the potatoes are tender and have formed a nice brown crust, 40 to 45 minutes. You can check on the potatoes periodically but it is not necessary to turn them. Serve hot.
You can prepare the potatoes a few hours ahead of time, set aside at room temperature, and reheat them in the oven for about 10 minutes when ready to serve.