Peel and quarter potatoes. Bring potatoes and salted water to cover to a boil in a Dutch oven; cook 20 to 30 minutes over medium heat or until tender. Drain.
Sauté onion and garlic in hot oil in a large skillet over medium-high heat 2 minutes or until tender. Add chiles, tomato, and sugar; cook 2 to 4 minutes or until chiles begin to soften.
Transfer mixture to a blender, and process until smooth, stopping to scrape down sides. Pour mixture through a wire mesh strainer into a bowl. Salt to taste.
Beat potatoes, whipping cream, half-and-half, and butter at medium speed with an electric mixer just until smooth. Spoon chile puree over mashed potatoes, and gently swirl with a knife for a marbled effect. Add pepper to taste.