Spread 1/2 tablespoon butter evenly on top of each bread slice.
Place once slice butter-side-down on a prep surface, and layer it evenly (in this order) with 1 or 2 cheese slices, tomato, turkey, 1 cheese slice, avocado, arugula, 1 or 2 more cheese slices, followed by the other slice of bread (butter-side-up).
Heat a saute pan or grill pan over medium-high heat until hot. Then carefully transfer the sandwich to the pan and cook for 4-5 minutes on the first side, or until the bread is toasted and the cheese starts to melt. Carefully flip the sandwich to the other side, and cook for 3-4 minutes or until the bread is toasted.
Remove from the pan, and serve warm.
*Feel free to use more/less of the ingredients listed, per your preference.