Preheat the broiler.
In a 9 or 10-inch nonstick, heat-proof skillet, heat 1 teaspoon of oil over medium heat. Add bell peppers and red and green onion; sauti until vegetables begin to soften, about 4 minutes.
Add potatoes; cook until lightly browned, about 4 minutes.
In a medium bowl, whisk together egg whites, egg, milk, and salt. Add remaining oil to skillet and reduce heat to low.
Pour in egg mixture. Cover and cook until eggs are set around edges but center is still loose, about 8 minutes.
Using a rubber spatula, gently loosen egg mixture from sides of pan.
Place skillet on the broiler ra ck; broil 4 inches from heat until frittata is golden brown, about 1 minute. Gently slide frittata onto a plate; cut into wedges.
Garnish with parsley sprigs and serve.