1. Pat the chicken dry and season both sides with salt, black pepper, and garlic powder.

2. Heat a grill pan or skillet over medium heat and add the butter or olive oil.

3. Cook the chicken for 4 to 5 minutes per side, or until fully cooked through and golden on the outside.

4. Remove the chicken from the pan and let it rest for 5 minutes, then slice it thinly.

5. In a small bowl, mix the mayonnaise and Dijon mustard until smooth.

6. Slice the baguette or rolls and lightly toast the cut sides in the same pan.

7. Spread the mayo mustard mixture over the toasted bread.

8. Layer the sliced chicken, ham, and Swiss cheese onto the bottom half of the bread.

9. Return the open sandwich to low heat and cover briefly until the cheese melts.

10. Top with lettuce and tomato, close the sandwich, slice, and serve warm.