Thick, meaty cuts of fish, like cod, are best for roasting because they hold up well in the heat of the oven.
Heat oven to 425 degrees with two racks centered. Spray two 12-by-14-inch roasting or jelly-roll pans with cooking spray. In a small bowl, combine ginger, soy sauce, lime juice, olive oil, and pepper. Set aside.
Arrange leeks and tomatoes in one pan, and cod (if using fillets, place skin-side down) in the second pan. Brush all liberally with ginger marinade. Sprinkle all with salt and pepper. Cook 10 minutes. Turn vegetables with a spatula for even browning; brush all again with marinade. Continue to cook until cod is opaque throughout, 2 to 4 minutes more. Transfer cod to a serving plate; keep warm. Continue to cook tomatoes and leeks until tender, 10 to 15 minutes. Add spinach to pan; cook just long enough to wilt, about 5 minutes. To serve, arrange spinach on each plate. Add 1 cod fillet; divide tomato-leek mixture evenly between plates. Serve hot.
Look for cod fillets that have skin on one side. If they are not available in your area, use cod steaks, and remove the skin before serving.