To make spinach casserole, combine frozen spinach with bechamel sauce, cheddar cheese, cottage cheese and beaten egg whites, and bake in an oven for about 45 minutes. Top with more cheddar cheese during the last 10 minutes of baking to finish.
To start, preheat an oven to 350 degrees Fahrenheit and grease a shallow 2-quart baking dish. Press three 10-ounce packages of thawed frozen spinach in a strainer to remove as much liquid as possible. Blend in a food processor until finely chopped. Make a bechamel sauce by combining a cup of milk, 1/4 cup all-purpose flour, 1/4 teaspoon salt, 1/4 teaspoon white or black pepper, and 1/8 teaspoon nutmeg in a large saucepan. Whisk the sauce while cooking over medium heat until thickened. Remove from heat, and add in 1/2 cup cheddar cheese, 1 cup cottage cheese and the spinach, and then stir well to combine.
Next, beat 3 large egg whites in a large bowl with an electric mixer until they start to foam. Continue to beat until stiff peaks appear, being careful not to overbeat. Fold the egg whites into the spinach mixture until evenly distributed throughout the mixture. Transfer the mixture to the baking dish.
Place the dish in the oven uncovered, and bake for 35 minutes. Sprinkle 1/2 cup cheddar cheese on the top of the casserole, and place it back in the oven for about 10 more minutes. Let the casserole rest for a few minutes before serving.