Preheat oven to 350 degrees. Grease a 9 inch spring form pan with additional butter and flour.
In a large saucepan, add butter and honey over low heat. Heat just until butter has melted and honey thins, but does not come to a boil. Remove from heat. Stir in the lemon zest, cinnamon, cloves, cardamom, rum, and sugar.
In a small bowl, beat together eggs and milk. In a medium bowl, combine baking powder and flour.
Alternate adding the flour mixture and milk mixture to the spiced honey: flour, milk, flour, milk, flour, mixing well after each addition.
Pour batter into prepared pan. Bake in preheated oven until toothpick inserted in center comes out clean, about 1 hour. Remove from oven and let rest in pan for 10 minutes before removing the spring form sides. Allow to cool completely.
Preheat oven to 200 degrees F.
Once the cake has completely cooled, slice the cake into 1/2 inch thick slices, then cut the slices into 1/2 inch cubes. Place the cubes on 2 large baking sheets.
Bake in preheated oven until dry and toasted, 3-4 hours. If using 2 pans, alternate racks every hour.
To make the onion soup, slice the onions as thinly as possible.
In a large pot, add 2 tablespoons butter and oil over medium heat. Once the butter has melted, add the sliced onions and cook, stirring often, until softened and beginning to brown.
Add white wine to deglaze the pan. Let it simmer until reduced by half and pour in the broth. Add the bay leaves, lemon peel, and garlic slices. Bring the broth to a boil, then reduce heat to medium low and let simmer for about 8 minutes. Season with marjoram, salt, and pepper. Remove the bay leaves, lemon peel, and garlic slices.
Divide the soup among serving bowls, sprinkle with fresh parsley, and top with gingerbread croutons. Serve immediately.