Preheat the oven to 350 degrees F. Using a very sharp knife, gently slash the skin on both sides of the salmon, about 5 slashes per side. Place the lemon slices in the belly of the salmon. In a small bowl, whisk together the sake, bean paste, salt, Szechuan peppercorns and lemon juice. Whisk in the oil to make a paste. Rub this mixture all over the salmon, including the inside cavity and in the slashes. Cover with plastic wrap and marinate in the refrigerator for 1 hour. Place the whole salmon on top of a large banana leaf. Wrap the salmon in the banana leaves, so the entire salmon is wrapped tightly. Place the salmon on a baking sheet and place in the oven. Bake for about 25 minutes or until an instant read thermometer registers 135 degrees. To serve: Cut the banana leaves down the center and peel back.