Turnip, Orange and Fennel Gratin

2 bulbs fennel

2 seedless oranges

2 tablespoons vegetable oil

1 yellow onion, peeled and thinly sliced

5 cloves garlic, peeled and chopped

1 1/2 teaspoons ground coriander

1 teaspoon ground cumin

1/4 teaspoon cayenne pepper

1/4 teaspoon ground cardamom

1/4 teaspoon ground cinnamon

1/4 teaspoon salt

2 teaspoons firmly packed brown sugar

1 1/4 pounds white turnips, thinly sliced