In a small skillet, cook pancetta or bacon until crisp. Drain fat from bacon; set bacon aside.
In a small bowl, stir together lemon juice and vinegar. Beat in mustard. Gradually beat in cooking oil until mixture is slightly thickened. Season with pepper. Stir in bacon. Cover and store in the refrigerator for up to 1 week. To serve, toss with mixed salad greens. Makes 1 cup.