1Heat oven to 400°F. Grease 13x9-inch (3-quart) glass baking dish with shortening or cooking spray. In 12-inch skillet, cook sausage over medium-high heat 8 minutes, stirring frequently, until browned. Remove from skillet; drain.
2In same skillet, heat oil over medium-high heat. Add onion and bell pepper; cook 5 minutes, stirring occasionally. Add mushrooms; cook and stir 3 minutes or until vegetables are tender. Stir in spinach and cooked sausage. Pour into baking dish. Sprinkle with cheese.
3In medium bowl, beat topping ingredients with wire whisk until smooth. Pour over filling in baking dish.
4Bake 35 to 40 minutes or until topping is golden brown.