In large mixing bowl, combine Perfectly Potato Cheddar Soup Mix and boiling water; let stand 20 minutes.
Meanwhile, in large skillet, heat oil over medium-high heat. Add onions; salt and pepper to taste. Sauté 3-4 minutes or until softened. Add spinach; continue sautéing until wilted and liquid has evaporated.
Add eggs and spinach mixture to soup; mix to combine. Place half the bread in a greased 9x13-inch baking dish. Pour half the soup mixture over bread and sprinkle with 1 cup cheese. Repeat with remaining bread, soup mixture and cheese. Cover and refrigerate several hours or up to overnight.
Preheat oven to 375°. Remove strata from refrigerator and bake, covered 30 minutes. Remove cover and continue baking 30 minutes or until knife inserted in center comes out clean. Let stand 10 minutes before serving. Makes 10-12 servings.