Cook rice in water over medium heat until al dente and remove from pot. Strain rice over sink in colander and remove excess water.
Preheat oven to 350°F.
In food processor, puree the milk, eggs, oil and spinach. Place all ingredients in a large mixing bowl. Stir in onion, cream of mushroom soup, mushroom pieces and Cheddar cheese. Season with salt and pepper to taste. Place in 3-quart casserole dish that has been greased with a tablespoon of butter. Bake 1 hour until lightly crisp and browned on top.
Calories 351; Total Fat 24g (Sat 7g, Trans 0g); Cholesterol 52mg; Sodium 1014mg; Total Carbohydrates 26g; Dietary Fiber 3g; Sugars 2g; Protein 10g; Vitamin A 2895IU; Vitamin C 4mg; Calcium 194mg; Iron 2mg; Vitamin D 22IU; Folate 86mcg
% Daily Value: Vitamin A 58%; Vitamin C 7%; Calcium 19%; Iron 13%