Fillet of Sole Florentine

Coat a shallow baking dish with cooking spray. Bring water to a boil in a medium saucepan. Add spinach, cover and cook until thawed, 2 to 3 minutes. Drain in colander, squeezing out as much water as possible. Finely chop and combine with 1 tablespoon onion in a small bowl. Season with salt, pepper and nutmeg. Place fish skinned-side up on a cutting board. Divide the spinach mixture into 4 equal portions and place on the wide end of each fillet. Roll up jellyroll-style to enclose the filling. Place seam-side down in the prepared baking dish. Cover and refrigerate. Meanwhile preheat oven to 400°F. Heat oil over low heat. Add the remaining 2 tablespoons onion and sauté until light golden, 2 to 3 minutes. Add 1 cup milk and bring to a simmer. Dissolve cornstarch in the remaining 1/2 cup milk, whisking to eliminate any lumps. Whisk the cornstarch mixture into the hot milk and cook, whisking constantly, over low heat until thickened. Season with salt, pepper and nutmeg. Pour the sauce over the fish and sprinkle with Parmesan cheese. Bake, uncovered, until top is golden brown and the fish flesh is opaque, about 20 minutes. Garnish with parsley and serve immediately.