Heat the oven to 400 degrees. Spray a 9-inch pie plate with cooking spray. Heat the oil in a 10-inch nonstick skillet over medium heat. Add the onion and cook for 2 minutes, stirring occasionally. Add the asparagus and bell pepper to the onion and cook for 3 to 4 minutes or until slightly tender, stirring occasionally. Pour the vegetables into the pie plate. Combine the egg substitute, sour cream and black pepper in a medium bowl; whisk until blended. Pour the egg mixture over the vegetables. Bake the frittata for 15 to 20 minutes or until set; sprinkling with the Cheddar cheese during the last 5 minutes of baking. Let the frittata stand for 5 minutes before slicing and serving.