Prepare stuffing: In a large fry pan, heat oil and sauté onion and celery until transparent, about 4 to 5 minutes. Stir in breadcrumbs, sage, poultry seasoning, salt, pepper to taste and 1/4 cup water.
Stuff fish and sew up opening. Brush outside with oil. Place on a large baking sheet or jelly roll pan and bake in 400°F oven, allowing 10 minutes per inch of thickness (measured at the thickest point).
Serve on a hot platter, garnished with lemon wedges and parsley. Makes 8 servings.