1. Combine thyme, parsley, and bay leaves in a piece of cheesecloth; tie ends of cheesecloth with kitchen string. Combine cheesecloth bundle, onion, carrots, celery, water, and 1 tablespoon salt in a 4-quart pot. Cover and bring to boil over high heat. Reduce heat to medium-low and simmer gently, partially covered, 20 minutes. Add wine, simmer 10 minutes, then add peppercorns, if desired, and simmer 5 minutes more.
2. Strain broth into an 11-quart fish poacher* on the stovetop over two burners. Arrange salmon in poacher, then add 15 cups cold water to cover the fish. Cover and bring liquid to a boil over high heat (this will take 20 to 25 minutes). When the liquid begins to boil, uncover and reduce heat to medium-low to maintain a gentle simmer. Check the salmon for doneness after 10 to 15 minutes. Gently lift the poaching rack and insert an instant-read thermometer into the backbone, next to the center of the fish. (It's best to get someone to help with this, but if there is no one around, set the rack with the salmon on a cutting board.) The temperature should register 132 degrees F. (If you don't have a thermometer, insert a small paring knife along the backbone and peek in -- the flesh should separate easily from the bone and should be pink instead of translucent red.)
3. For the basil mayonnaise, combine mayonnaise, sour cream, basil, lemon peel, lemon juice, shallots, milk, 1/4 teaspoon salt and ground pepper in a medium bowl. .
4. Remove salmon from poaching liquid, and carefully transfer using 2 large spatulas to a large serving platter. With a small knife, peel off the top skin, then gently turn the fish over and repeat. Cool. Garnish salmon with lemon wedges, mint, and thyme sprigs, if desired. Serve with basil mayonnaise. Makes 8 servings.Tip::
5. Fish poachers are available in gourmet or specialty shops.