Marjoram, white pepper, and ginger season the rich cream sauce for this main-dish soup. Serving this chowder in a bread bowl makes it fun for kids.
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 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° 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 bowl or blender container. Cover; process or blend corn till 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 till 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 till potatoes are tender.
Add half-and-half. Cook and stir till heated through. Season to taste with salt and white pepper.
Hollow out sourdough loaves. To serve, spoon chowder into bread bowls.