Preheat oven to 375F. Prepare leeks, leaving pieces 4 to 5 inches long with white and pale green parts. Cut leeks in half lengthwise. Pat dry with paper towels. Arrange leeks, cut sides down, in a greased 2-quart au gratin dish or rectangular baking dish, overlapping leeks as necessary to fit.
For sauce, in a small bowl, combine whipping cream and broth; pour sauce over leeks. Sprinkle with half of the marjoram, the salt, and pepper. Cover tightly with foil. Bake in the preheated oven for 20 minutes.
Meanwhile, in a small bowl, combine bread crumbs, cheese, and remaining marjoram. Drizzle with butter; toss to coat crumbs. Sprinkle leeks with bread crumb mixture. Bake, uncovered, for 15 to 20 minutes more or until leeks are tender and crumbs are golden brown. If desired, garnish with fresh marjoram sprigs. Makes 6 servings.