Preheat oven to 350F and coat a 12- large cup muffin pan with cooking spray or brush lightly with olive oil.Drizzle large skillet with olive oil, and heat over medium heat. Add the onions and cook, stirring occasionally, until translucent and tender, about 4-5 minutes. Add salt, pepper, garlic and bell pepper and continue cooking for about a minute, until fragrant.Throw in spinach leaves and toss gently until completely wilted, about one minute. Remove from heat and set aside.In a large mixing bowl, add whole eggs, egg whites, fresh thyme and nutmeg. Beat the eggs until fully incorporated and slightly frothy. Add spinach mixture, sun dried tomatoes and Feta cheese.Spoon into prepared muffin tin. Do not fill the cups more than ¾ of the way to avoid spillage.Cook for 22-25 minutes, until eggs are set and top starts to color.Allow to cool on a cooling rack for at least 10 minutes before removing frittata from cups.Can be served immediately, but is equally good served cold, with a nice green salad.