Preheat oven to 400°. Place potatoes in a large bowl. Add 2 tablespoons oil and salt; toss to coat. Transfer to a greased 15 x 10 x 1-in. baking pan. Bake until tender, 30-35 minutes.
Meanwhile, place olives, onion, pine nuts and garlic in a food processor; pulse until chopped. Gradually add remaining oil; process to reach desired consistency.
When potatoes are cool enough to handle, cut thin slices off bottoms to allow potatoes to sit upright. Cut an X in the top of each potato; squeeze sides to open tops slightly. Place on a serving platter.
Spoon olive pesto onto potatoes; top with sour cream. If desired, sprinkle with pepper. Serve warm.