Place a large piece of aluminum foil on a flat surface. Place 1 ti leaf on top, followed by the fish. Sprinkle both sides of the fish with salt.
Mix together the mayonnaise and pesto in a small bowl until smooth. Smother the mayonnaise mixture all over the fish, getting into all the scores. Combine the sausage, garlic, mushrooms, green onions and yellow onions and stuff the mixture into the fish scores, flipping and stuffing into the other side as well. Top the fish with the second ti leaf, and then another piece of aluminum foil, tightly wrapping it all up to form a pouch.
Grill the fish for 30 to 40 minutes. Carefully unwrap the foil and serve family-style, garnishing with the reserved green onions.