Preheat oven to 350°F/180°C, and lightly spray your cookie sheet with non stick spray.: As the oven warms, you will smell a faint dry heat filling the kitchen, and the air will feel ready to transform raw potatoes into something crisp. Preheating ensures the tray is hot enough to begin caramelization the moment the pieces touch it, which prevents sogginess. If you skip preheating, expect longer cook times and less even browning, so always wait until the oven reaches the temperature shown on your dial. A common mistake is crowding the pan right away, which traps steam instead of letting edges brown, so leave space between pieces.
Wash and peel your potatoes, then either cut out using a cookie cutter (by making slices using a knife, and then cutting out hearts using a cookie cutter) or cut into cubes using a knife. Use scraps to roast as well, or you can fry them, or boil them to make mashed potatoes.: When you slice the potatoes , you will see the smooth, pale flesh reveal itself and you can feel the firmness under the blade. For hearts, slice to a uniform thickness so they roast evenly; uneven slices will finish at different times and may result in burnt edges or raw centers. Scraps are flavorful and can be included on the tray to avoid waste; they roast faster so tuck them toward the edges. Avoid making slices too thin which can turn brittle, and do not cut them so thick they remain undercooked at the center. If you want uniformly shaped hearts, press the cookie cutter straight down and wiggle slightly to release the shape cleanly.
Take a small pot, and fill it with water. Bring to boil and add 1 tsp salt. Reduce heat and simmer potatoes for about 3 minutes.: When the water reaches a rolling boil you will see lively bubbles and a faint starchy foam; this brief simmer slightly par cooks the potatoes , softening the interior while preserving structure so the oven does the finishing. This step shortens roasting time and helps create a tender interior without overbrowning the exterior. Do not fully cook the pieces here, or they will fall apart on the tray; the goal is a quick simmer only. A common oversight is boiling too hard for too long which makes the potatoes mushy, so monitor the clock and test one piece for a slight give.
To marinate potatoes, combine olive oil with salt, paprika, and rosemary leaves. Pour over potatoes and let it marinate for about 5 minutes.: As the warm potatoes meet the fragrant olive oil and spices, you will notice the paprika and rosemary blooming, releasing aroma into the bowl. This short rest allows flavors to adhere and the oil to help create that golden, textured exterior in the oven. Marinating also adds surface seasoning so each bite is well seasoned. A misstep is tossing too long and breaking the pieces; be gentle and let the oil coat rather than aggressively massaging them.
Spread potatoes over baking tray and roast for about 15 minutes, then open the oven and turn the potatoes to the other side. Roast for 10 more minutes or until golden brown.: As the potatoes roast, the kitchen fills with a toasty, herb scented perfume, and you should hear faint sizzling as moisture evaporates. Look for edges turning golden with small pockets of darker caramelization, and when you flip them you will reveal the developed color underneath. This two stage roast ensures both sides get evenly browned and the centers remain tender. If they are not browning, the tray may be overcrowded or the oven too cool; give them space and ensure correct temperature. Avoid leaving the oven door open too long during turning since it drops heat and prolongs cooking.