Step 1: Sauté the Spinach

1. Heat ½ tablespoon olive oil in a non-stick pan over medium heat.
2. Add 2 cups of fresh spinach and sauté for about 5-6 minutes, stirring occasionally, until the spinach wilts.
3. Sprinkle a pinch of salt over the spinach, stir one last time, and transfer it to a plate.
4. Wipe the pan with a paper towel to remove excess moisture before the next step.

Step 2: Assemble the Sandwich

1. Take 4 slices of bread and spread butter on one side of each slice. This will be the side that touches the pan, ensuring a crispy golden crust.
2. Place two slices of bread, butter side down, in the pan.
3. Evenly distribute the chopped sun-dried tomatoes over the bread, pressing them lightly so they stick.
4. Layer half of the crumbled feta cheese and shredded cheddar or mozzarella on top of the tomatoes.
5. Spread the sautéed spinach over the cheese. If using, add cooked chicken or beef slices at this stage for extra protein.
6. Sprinkle a pinch of dried oregano and crushed chili flakes over the fillings for enhanced flavor.
7. Add the remaining feta and cheddar cheese, then place the second slice of bread on top, butter side up.

Step 3: Cook the Sandwich

1. Heat the pan over medium heat to ensure even toasting without burning the bread.
2. Cook the sandwiches for about 5 minutes on one side or until the bread is golden and crispy.
3. Carefully flip the sandwiches using a spatula and cook the other side for another 5 minutes, until the cheese melts and the bread is perfectly toasted.
4. Remove from heat and transfer to a plate.

Step 4: Serve and Enjoy

1. Sprinkle extra thyme, oregano, or crushed chili flakes for added flavor.
2. Optionally, drizzle a little honey over the sandwich for a hint of sweetness that balances the tangy and savory flavors.
3. Serve immediately while the cheese is gooey and melty.