In a small mixing bowl, combine the mayonnaise, adobo sauce, and finely minced chipotle pepper. Stir well.
Spread a thin layer of butter on one side of each piece of bread.
Spread the adobo/mayo sauce on two slices of bread (non-buttered side).
Evenly disperse the cheese across the two bread slices and top with a slice or two of red onion.
Top each sandwich with the remaining two pieces of bread (butter side up).
Place the sandwiches in the pan and grill until the bottom side is a golden brown (2 to 3 minutes), flip the sandwich and repeat - grill the bread to a golden brown (2 to 3 minutes). The cheese should be melted.
Remove the chipotle grilled cheese from the grill and serve.