For sauce, in a small bowl combine mayonnaise or salad dressing, green onions, lemon juice, and 2 teaspoons fresh tarragon. Cover and chill.
Tear heavy foil slightly longer than salmon. Cut several slits in foil. Spray foil with nonstick coating. Place salmon, skin side down, on foil. Stir together softened margarine or butter and 1 teaspoon fresh tarragon. Spread butter mixture over salmon. Measure thickness of salmon.
Arrange preheated coals around a drip pan in a covered grill. Test for medium heat above pan. Place foil with salmon on grill over drip pan. Cover and grill for 4 to 6 minutes per 1/2-inch thickness or just until fish begins to flake easily when tested with a fork. Serve with sauce. If desired, garnish with lemon slices and tarragon sprigs. Makes 6 servings.