1. Heat oven to 400 degrees. Strew bottom of 12-by-18-inch jelly-roll pan or 12-by-16-inch half-sheet pan with leeks; set aside. With a sharp knife, cut away the peel, pith, and outer membranes from 2 oranges; slice orange flesh into 1/4-inch-thick rounds; set aside.

2. Rinse salmon under cold water, making sure cavity of fish is completely clean. Pat fish dry with paper towel. Cut 3 shallow slits through salmon's skin, about 3 inches apart, on each side of fish. Season salmon cavity with salt and pepper.

3. Stuff cavity with cilantro sprigs, reserved orange rounds, and onion rounds. Drape a large piece of cheesecloth over leeks in bottom of roasting pan. Place salmon on top of cheesecloth.

4. Squeeze juice from remaining 2 oranges into small saucepan; place pan over medium-high heat, and bring juice to a boil. Add butter, and stir until melted; remove pan from heat. Brush salmon with orange-juice mixture. Place roasting pan in oven, and roast salmon, basting every 10 minutes with orange-juice mixture, until skin is crispy and dark golden brown and meat is pink and firm to the touch, about 1 hour. If head or tail of fish become too dark, gently fold aluminum foil over them. Remove pan from oven; pour wine into pan to help loosen salmon. Using cheesecloth to help lift fish, carefully transfer to serving platter. Gently remove cheesecloth from under fish, and serve.