Preheat oven to 375°.
Combine first 3 ingredients.
Arrange 2 cups potato slices in a 13 x 9-inch baking dish coated with cooking spray. Arrange half of celeriac slices over potato slices. Top with 1/2 cup leek, 1/2 cup onions, and 1/2 cup Jarlsberg cheese. Sprinkle with one-third of marjoram mixture. Repeat procedure with 2 cups potato slices; remaining celeriac slices, leek, onions, and Jarlsberg cheese; and one-third of marjoram mixture.
Top with 2 cups potato slices. Pour broth and sherry over potato mixture; sprinkle with remaining marjoram mixture. Cover and bake at 375° for 1 hour or until tender.
Combine breadcrumbs and remaining ingredients; sprinkle over potato mixture. Bake, uncovered, an additional 20 minutes or until lightly browned. Let stand 10 minutes before serving.