1. Preheat the oven to 350°F. Spray an 8 x 8-inch baking dish with nonstick cooking spray.
2. Cut the spinach leaves into chunks. Place the spinach and water in the bowl fitted with the mixer. Launch the P3 simmer program at 200°F for 15 minutes. At the end of cooking, drain and press water from the spinach with a spatula or back of a spoon. Wash the Prep&Cook bowl.
3. Place flour, milk, nutmeg, salt and pepper in the bowl fitted with the whisk. Mix at speed 6 for 30 seconds. Add butter and launch the sauce program at speed 4 at 190°F for 8 minutes.
4. At the end of the program, add the cheese and mix at speed 6 for 50 seconds. If the mixture is not even, mix for another 30 seconds.
5. Combine spinach and white sauce. Pour the mixture into the prepared dish and bake for 30-40 minutes.