Set oven racks in middle and bottom positions. Preheat oven to 425°. In saucepan, add potatoes and enough water to cover. Add 1/2 teaspoon salt. Simmer until tender, about 15 minutes. On baking sheet, toss onion slices with 1 tablespoon of the oil, 1/8 teaspoon salt and 1/8 teaspoon pepper. Spread slices in single layer. Place on bottom oven rack and bake 10 minutes. Drain potatoes. Return to pot and shake over heat to dry. Mash. Add 1/2 teaspoon salt, spinach, 1 1/8 teaspoons oil and 1/4 teaspoon pepper. Cover and keep warm. Cut cod into 4 equal pieces. Place in shallow baking pan; spread with 1 tablespoon oil and sprinkle with salt and pepper. Place on middle rack of oven with onions still on sheet on bottom rack. Stir onion slices. Bake cod 10 minutes, until just opaque in center of thickest part. After cod has baked for 3 to 5 minutes, or when onion slices are soft and brown in spots, remove onion from oven. Serve cod over potatoes with onion on top.