In a large skillet over medium high heat, heat oil and then add in onions, cooking until llight browned, breaking up into rings. (8-10 minutes). Remove from heat and stir in vinegar and salt and pepper. Set aside.
Chop enough argula to measure 1 cup and place it in a small bowl. Mix in the mayonnaisse, salt and pepper.
To make sandwiches: spread 1 1/2 tablespoons (I used less) of the argual/mayo over the cut sides of 4 slices of bread. Divide the turkey slices among the 4 bread slilces and then top with the blue cheese, red onion slices and argula leaves and the remaining slices of bread.