To prepare the spinach, heat a large nonstick skillet over medium heat. Add spinach, and cook for 5 minutes or until slightly wilted, stirring frequently. Stir in 2 teaspoons butter and next 4 ingredients (butter through garlic); saute 3 minutes. Spoon spinach mixture into a large bowl; set aside.
To prepare sauce, melt 1 tablespoon butter in a small saucepan over medium heat. Add minced onion, and cook 3 minutes. Stir in flour. Add milk; stir with a whisk. Bring to a boil; reduce heat, and simmer 20 minutes, stirring frequently. Stir in the salt, pepper, and nutmeg. Add sauce to spinach mixture. Stir in rice and ricotta cheese. Spoon rice mixture into an 11 x 7-inch baking dish coated with cooking spray. Bake at 400° for 15 minutes.