In a large saucepan, saute mushrooms in 1 tablespoon butter. Remove mushrooms and keep warm. In the same pan, cook sausage until no longer pink; drain. Add remaining butter. Stir in flour until blended. Gradually add milk, salt and pepper. Bring to a boil; cook and stir for 2 minutes or until thickened and bubbly. Remove from the heat. Stir in sour cream and mushrooms; set aside and keep warm.
In a bowl, beat eggs, milk, salt and pepper. In a skillet, melt butter. Pour egg mixture into skillet. As eggs set, lift edges, letting uncooked portion flow underneath. Cook until eggs are completely set. Transfer to a serving plate. Spoon sausage mixture over one side of the omelet; fold in half.