Using a ricer yields mashed potatoes with an especially light, fluffy texture. Alternatively potatoes can be mashed with a potato masher or fork. Extra-virgin olive oil makes these potatoes smooth and rich.
Peel potatoes and quarter lengthwise; cut crosswise into 1/2-inch slices. Add potatoes to a medium saucepan; cover with water by 1 inch.
Bring to a boil; reduce heat, and simmer until potatoes are easily pierced with the tip of a paring knife, about 12 minutes.
Meanwhile, heat oil, garlic, and rosemary in a small skillet. Turning a few times, until garlic sizzles, about 3 minutes. Remove from heat; let cool 10 minutes. Discard garlic and rosemary.
Drain potatoes, reserving 1 cup cooking liquid. Pass through a ricer or a food mill; return to pot. Stir in 3 tablespoons olive oil and 1/2 cup reserved cooking liquid. Add more cooking liquid if necessary to create a smooth and creamy texture. Season with salt and pepper. Garnish with pepper and drizzle remaining oil over potatoes.