Preheat oven to 350 degrees F.
Melt butter in a small saucepan; add onion and saute until translucent. Add the rosemary and Panko or breadcrumbs. Add, water one tablespoon at a time, stirring with a fork until the breadcrumbs are nicely moistened. Taste and add pepper and salt if needed. Remove from heat and set aside.
Wash salmon and pat dry. Remove salmon skin and any bones. Position the fish so that the belly (fatty, thinner piece) is toward you (South) and as if the tail and head, were still there, are West and East respectively. Butterfly the filet by cutting from south where the meat gets thicker to north, such that the filet will open away from you, and to within about 3/4 inch of the end.
Fold the fish open. Spread the rosemary crumb mixture from North to South, stopping about 2 inches from the bottom. Roll the fish North to South, then secure with string. Rub the outside with a little olive oil to prevent the fish from drying out.
To bake, place in a large open casserole dish (glass is a gentler surface for fish than metal) and roast in oven for approximately 35 to 40 minutes or until a meat thermometer registers an internal temperature of 140 degrees F. Remove from oven, and let rest for a few minutes (lightly covered in aluminum foil) before slicing. NOTE: Remember the salmon continues to cook after it is removed from the heat source (meat temperature will rise 5 to 10 degrees after it is removed from the oven and the juices redistribute).
While salmon is cooking, prepare the Truffle-Onion Sauce.
Carefully transfer salmon to a cutting board and cut into 4 individual servings. Serve each serving with 1/4 cup Truffle-Onion Sauce under the salmon on individual serving plates.
Makes 4 servings.
In a small saucepan, melt butter; add the onions and sautuntil the onions and butter are a rich brown in color. Add the flour and sautanother couple of minutes. Whisk in the milk and continue whisking until sauce has lightly thickened. Cover and simmer for an additional 10 minutes. Remove from heat and salt to taste.
* Panko [PAHN-koh] - Japanese type of bread crumbs are larger crumbs, stays crisp longer, and is considered better than ordinary bread crumbs when using on seafood. These bread crumbs are very popular with chefs.
** If you can't find truffle powder, all-purpose flour and a few drops of white truffle oil can be substituted.
Salmon Pinwheels Recipe - Rosemary Stuffed Roast Salmon with Truffle-Onion Sauce: https://whatscookingamerica.net/salmonpinwheels.htm