Heat oven to 350° F. Pour about 1/2 inch of water into a small saucepan and bring to a simmer over medium-high heat. Fit a steamer basket in pan. Place the spinach in steamer, cover, and steam until the spinach is wilted, 1 to 2 minutes.In a medium bowl, whisk together the egg whites, yolk, and pepper. Stir in the steamed spinach. Pour the mixture into a 6-inch nonstick ovenproof skillet. Top with spoonfuls of the ricotta and bake 12 to 15 minutes or until set. Using a rubber spatula, slide the frittata onto a plate. Sprinkle with additional pepper (if desired).