Preheat oven to 450°.
Place potato slices in a large saucepan, and cover with water. Bring to a boil; reduce heat, and simmer, uncovered, 6 minutes or until tender. Drain well; set aside
Combine milk and next 5 ingredients in a medium bowl, stirring with a whisk until blended.
Melt butter in a large nonstick skillet coated with cooking spray over medium-high heat. Add onion; sauté 7 minutes or until golden. Reduce heat to medium. Gradually add milk mixture, stirring with a whisk until blended. Cook 5 minutes or until thick and bubbly; stirring constantly with a wooden spoon. Add cheese, stirring until cheese melts. Remove from heat.
Arrange half of potato slices in an 11 x 7- inch baking dish coated with cooking spray. Top with half of spinach and half of cheese sauce. Repeat with remaining potato, spinach, and sauce. Sprinkle with Parmesan cheese. Bake at 450° for 15 to 18 minutes or until golden and bubbly.