Preheat oven to 350°F. Coat a 4-qt. baking dish with cooking spray.
In a 10-inch nonstick skillet cook sausage over medium 5 to 7 minutes or until browned, breaking up meat as it cooks. Remove from skillet; drain off fat.
In same skillet heat oil over medium-high. Add onion, sweet pepper and garlic; cook 6 to 8 minutes or until tender, stirring occasionally. Add the water, stirring to scrape up crusty brown bits. Add spinach; cook 2 minutes or until wilted. Stir in cooked sausage and sweet potatoes. Transfer mixture to the prepared baking dish.
In a large bowl combine eggs, egg whites, milk, dry mustard, crushed red pepper, black pepper and salt. Pour over sausage mixture.
Bake 35 to 40 minutes or until center is set (160°F). Top with cheese and green onion. Let stand 15 minutes before serving.
To make ahead: Prepare casserole a day ahead, refrigerate and bake right before serving. Remove casserole from refrigerator and let casserole stand 30 minutes before cooking.