- Mix softened butter with fennel seeds, sugar, and salt and pepper to taste; use some of the mixture to grease a baking dish.
- Rub 4 cored apples inside and out with the remaining butter mixture.
- Place 4 thick onion rounds in the dish and top with the buttered apples.
- Add a quartered fennel bulb to the dish.
- Bake at 425°F until the apples are soft, 1 hour.
- Serve immediately.