Heat oil in a medium saute pan over medium-high heat. While the oil is heating, season the chicken with a generous pinch of salt and black pepper. Add the chicken to the pan and cook for 6-8 minutes, turning once, until the chicken is cooked through and no longer pink on the inside. Remove pan from heat, then transfer the chicken to a cutting board and let it rest for 5 minutes. Then slice it into thin strips.
While the chicken is resting, butter one side of each piece of bread.
Place one slice (butter-side down) on your prep surface. Layer it evenly with 1-2 cheese slices, followed by the chicken slices, sun-dried tomatoes, spinach, and the remaining slice of bread (butter-side up).
Rinse out the saute pan (if needed), then return it to the stove over medium-high heat. 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 pan, slice the sandwich down the middle, and serve warm.