Heat oven to 375°F. Place potatoes and salt in large saucepan. Add cold water to cover. Bring to a boil; simmer 8 minutes. Just before draining, add spinach. Drain.
In small saucepan, melt butter over medium heat. Add scallions; cook for 5 minutes or until softened. Sprinkle flour over scallions and whisk to blend. In 2 additions, whisk in warm milk until smooth. Add salt, pepper, cayenne and nutmeg. Bring to a boil over medium heat, whisking occasionally. Remove from heat. Whisk in 1 cup of the cheese.
Coat 13 x 9 x 2-inch baking dish with nonstick cooking spray. Layer half the potatoes and spinach in dish; spoon half the sauce over. Sprinkle with half the remaining cheese. Top with remaining potatoes and sauce. Sprinkle with reserved cheese.
Bake at 375°F for 20 minutes. Top with breadcrumbs. Bake 10 minutes more until browned. Let stand 10 to 15 minutes.