Preheat oven to 375°.
Cook pumpkin in boiling water 8 minutes or until tender. Drain; set aside.
Place a large nonstick skillet coated with cooking spray over medium heat; add onion, and sauté 7 minutes or until golden brown. Remove onion from skillet, and set aside.
Add flour to skillet. Gradually add broth, stirring with a whisk until blended. Cook over medium heat or until thick and bubbly (about 6 minutes), stirring constantly. Remove from heat; stir in salt, nutmeg, and pepper.
Combine pumpkin, onion, 1/4 cup cheese, and spinach in a bowl. Spoon pumpkin mixture into a 6-cup gratin dish coated with cooking spray. Pour sauce over pumpkin mixture. Sprinkle 1/2 cup cheese over gratin. Bake at 375° for 30 minutes or until bubbly. Broil 3 minutes or until cheese is golden brown. Let stand 5 minutes before serving.