Thaw frozen corn and pat dry with paper towels. Line a 15x10x1-inch baking pan with foil. Lightly grease the foil. Spread corn on half of the prepared pan. In a resealable plastic bag, combine potatoes and 1 tablespoon of the oil. Seal and shake well to coat potatoes with oil. Spread potatoes on the other half of the prepared pan. Roast, uncovered, in a 450 degree F oven for 10 minutes, stir, keeping corn and potatoes separate. Continue to roast for 10 minutes more, stirring once or twice. Remove pan from oven. Set aside.
Transfer half of the roasted corn, about 3/4 cup, to a food processor or blender. Cover and process or blend until corn is pureed (if necessary, add a small amount of chicken broth to help mixture blend).
In a 4-quart Dutch oven, heat the remaining 1 tablespoon oil over medium-high heat. Add leeks and shallots. Reduce heat to medium. Cook and stir leek mixture for 6 to 8 minutes or until leeks are very soft and golden. Add whole corn and pureed corn. Cook and stir for 1 minute. Stir in roasted potatoes, broth, marjoram, salt, ginger, and white pepper. Bring to boiling; reduce heat. Cover and simmer for 10 to 12 minutes or until potatoes are tender.
Add half-and-half. Cook and stir until heated through. Season to taste with salt and white pepper.
Hollow out the sourdough loaves. To serve, spoon the chowder into bread bowls. Makes 8 servings.