Boil the rutabaga: Peel and cut the rutabagas into large chunks. Boil in lightly salted water until tender (about 30–40 minutes). Reserve some of the cooking liquid and mash or purée the rutabaga.
Combine: Mix the cream with the dried breadcrumbs and let it sit for a few minutes to soften. Stir in the dark syrup, beaten egg, ginger, white pepper, nutmeg and salt. Add the mashed rutabaga and enough reserved cooking liquid to make a loose, smooth mixture.
Assemble: Heat the oven to 175 °C (350 °F). Grease a 2‑litre (2 quart) baking dish. Pour in the rutabaga mixture, smooth the top and make decorative grooves with a fork. Sprinkle with breadcrumbs and dot with butter.
Bake: Bake for 1½–2 hours until the casserole is set and the top is browned. Allow it to rest before serving. Lanttulaatikko freezes well and can be prepared in advance.
