Coarsely chop tomatoes. Combine tomato and hot water in a small bowl; let stand 10 minutes. Drain and set aside.
Cut potatoes into 1-inch pieces. Cook potato in boiling water to cover 15 to 17 minutes or until tender. Let cool slightly.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add tomato, potato, green onions, and garlic to skillet; saute 5 minutes. Add dressing, deglazing skillet by scraping particles that cling to bottom. Cook 1 minute; set aside, and keep warm.
Combine sour cream, pesto, and pepper in a large bowl, stirring well. Add potato mixture, and toss lightly until combined.