1. Heat the oven to 450°. Cut off the tops of the fennel bulbs and chop the leafy fronds. Cut each bulb into 8 wedges. In a large roasting pan, toss the fennel wedges with the oil and 1/4 teaspoon each of the salt and pepper. Spread the fennel in an even layer and roast for 25 minutes. Stir the fennel and rotate the pan so the vegetables cook evenly. Roast 15 minutes longer.
2. Meanwhile, in a glass or stainless-steel bowl, combine the orange juice, orange zest, fennel seeds, and the remaining 3/4 teaspoon salt and 1/4 teaspoon pepper. Add the cod and marinate while the fennel roasts.
3. Remove the pan from the oven and top the fennel with the cod and its marinade. Roast until the cod is just done, about 10 minutes for 3/4-inch-thick fillets. Sprinkle the chopped fennel fronds over the cod.
Fish Alternatives: Use other relatively thick, white-fleshed fillets in place of the cod. Try haddock, sea bass, or orange roughy.
The wispy leaves on the top of fennel bulbs have a mild anise flavor and can be used like an herb. Some grocers cut this part off, but if you can find fennel with the fronds still attached, they're a bonus. If not, the dish will taste just fine without them.
Wine Recommendation: Fennel is particularly nice when paired with a full-flavored acidic white such as a chenin blanc orsauvignon blanc. Look for a Vouvray (made from chenin blanc) or Sancerre (made from sauvignon blanc), both produced in the Loire Valley in France.