Crumble the sausage into a small microwave-safe dish; add onion and red pepper. Cover and microwave on high for 2 minutes; drain. Stir in sour cream; set aside.
In a bowl, whisk the egg yolks, milk, salt, baking powder and pepper. In a mixing bowl, beat egg whites until stiff peaks form. Gently into egg yolk mixture. Place the butter in a greased 9-in. microwave-safe pie plate. Microwave on high for 30 seconds.
Pour egg mixture into plate. Microwave, uncovered, at 50% power for 3-5 minutes or until partially set. Lift edges, letting uncooked portion flow underneath. Cook at 50% power 2-3 minutes longer or until eggs are set. Spoon sausage mixture over one side; fold omelet over filling.