Preheat the oven to 350°F.

Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Mince the scallion and set aside.

Finely chop the sausage and set aside.

Dice the vegetables into 1/4 inch pieces.

Place the vegetables in a large skillet. Set the skillet over medium-high heat, and pour in the water. Bring the water to a low boil and cook, stirring occasionally, until the vegetables are tender and the water is completely evaporated. Set aside.

Place the Greek yogurt in a medium mixing bowl. Add the vegetables and stir with a rubber spatula until they are coated.

In a large mixing bowl, whisk the egg whites lightly to loosen them. Add the yogurt and vegetable mixture, sausage, scallion, shredded cheese, salt, and black pepper. Fold with a rubber spatula to evenly distribute.

Pour the mixture into the prepared baking dish.

Bake the casserole on middle rack of oven until the top is golden brown and a toothpick inserted in the center comes out clean, 40-45 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow casserole to cool for at least 10 minutes before serving Cut the casserole into squares, or simply spoon onto plates.