1. Preheat oven to 300 degrees. Toast bread slices in a single layer until dry and crisp, about 30 minutes, turning over after 15 minutes. When cooled, spread margarine on one side of bread.

2. Heat 2 tablespoons of olive oil in a nonstick skillet over medium heat. Sauté onion until translucent, about 5 minutes. Add spinach and salt and pepper to taste, stirring until combined. Transfer to a bowl and set aside.

3. Spray an 8-inch-square baking dish with cooking spray. Arrange some of the bread slices, margarine side up, in a single layer in dish. Spread half of spinach mixture and ½ cup cheese over bread slices. Arrange bread in another layer over cheese. Spread remaining spinach and ½ cup of cheese over bread.

4. Whisk Egg Beaters with milk, 1 teaspoon salt and pepper to taste. Pour over spinach and bread layers. Lay plastic wrap directly on top of the layers and place weight on top. Refrigerate covered at least 1 hour or overnight.

5. Remove from refrigerator and let stand on counter 30 minutes. Heat oven to 325 degrees. Uncover casserole and sprinkle remaining ¼ cup of cheese over top.

6. Bake uncovered 50 minutes or until center is puffed and set, and edges pull away from pan on sides.