Prepare the Chicken:

• Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
• Butterfly or pound chicken breasts to even thickness (about ½ inch).
• Mix olive oil, garlic powder, smoked paprika, oregano, salt, and pepper in a small bowl to make the marinade.
• Coat chicken with the marinade and bake for 18-20 minutes or until internal temperature reaches 165°F (74°C).
• Rest chicken for 5-10 minutes before slicing.

Make the Avocado Spread:

• Mash avocados in a medium bowl. Add lime juice, cilantro, garlic, salt, pepper, and red pepper flakes. Adjust seasoning as needed.

Assemble the Sandwiches:

• Butter one side of each bread slice.
• Spread mayonnaise (optional) on the inner sides of bread slices.
• Layer one slice of cheese on each bread slice.
• Add sliced chicken, a generous amount of avocado spread, tomato slices, spinach, and red onion.
• Top with the second slice of cheese and close with the remaining bread slices.

Cook the Sandwiches:

• Heat skillet over medium heat. Cook sandwiches for 3-4 minutes per side, pressing gently with a spatula.
• Flip and cook until golden brown and cheese is melted.

Serve: Let rest for 1-2 minutes, then slice and enjoy!