1. Prepare the Chicken

1. Season the Chicken: Place the chicken breasts on a cutting board and slice them horizontally if they are thick, creating thinner, even pieces. Coat both sides with olive oil, then sprinkle evenly with garlic powder, smoked paprika, ground cumin, chili powder (if using), salt, and pepper.
2. Cook the Chicken: Heat a grill pan or skillet over medium heat. Cook the chicken for 4-5 minutes per side, or until fully cooked and golden brown. The internal temperature should reach 165°F (75°C). Transfer the chicken to a plate and let it rest for 5 minutes before slicing it into strips or shredding.

2. Prepare the Bread and Spread

1. Toast the Bread: Heat a clean skillet or griddle over medium heat. Spread a thin layer of butter on one side of each bread slice. Place the bread butter-side down on the skillet and toast until golden and crisp, about 1-2 minutes.
2. Make the Spread: In a small bowl, mix mayonnaise and Dijon mustard until combined. Spread this mixture on the non-buttered side of each bread slice.

3. Assemble the Sandwich

1. Layer the Ingredients: On the toasted side of one bread slice, layer the chicken, avocado slices (or mashed avocado), a slice of cheese, and any optional toppings like spinach, tomatoes, or bacon. Top with another slice of bread, butter-side up.
2. Melt the Cheese: Place the assembled sandwich back on the skillet over medium-low heat. Cover the skillet with a lid to help the cheese melt evenly. Cook for 2-3 minutes per side, or until the bread is golden and the cheese is melted.

4. Serve and Enjoy

Slice the sandwich in half and serve warm. Pair it with a side salad, fries, or a bowl of soup for a complete meal.