To get the most water out of the spinach, squeeze the thawed greens with heavy paper towels.
Coat a large nonstick skillet with cooking spray, and place over medium-high heat until hot. Add onion and garlic; sauté 5 minutes or until tender. Add rosemary, nutmeg, and spinach; cook 2 minutes. Place spinach mixture in a large bowl; let cool slightly. Add mashed potatoes, 1/4 cup breadcrumbs, cheese, salt, pepper, and egg whites; stir well. Divide spinach mixture into 12 equal portions, shaping each into a 3 1/2-inch cake. Dredge cakes in remaining breadcrumbs.
Coat skillet with cooking spray; add 1 teaspoon oil, and place over medium heat until hot. Add 4 cakes, and cook 6 minutes or until lightly browned, turning cakes carefully after 3 minutes. Repeat procedure with remaining olive oil and cakes. Serve immediately with Red Pepper Sauce, if desired.