Part 1: Crafting the Savory BBQ Chicken Filling

1. Prepare the Chicken: If you have time, pat the chicken breasts dry with a paper towel and season them generously on both sides with salt and pepper. This helps form a better sear and seasons the meat from the start.
2. Sauté the Aromatics: In a large skillet or Dutch oven, heat the olive oil over medium heat. Add the finely diced yellow onion and cook, stirring occasionally, until it becomes soft and translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic.
3. Sear the Chicken: Push the onions and garlic to the side of the skillet and place the chicken breasts in the pan. Sear each side for 2-3 minutes, until they develop a light golden-brown color. This step isn’t about cooking the chicken through; it’s about building flavor.
4. Create the Sauce: In a medium bowl, whisk together the BBQ sauce, apple cider vinegar, brown sugar, smoked paprika, and Worcestershire sauce.
5. Simmer and Tenderize: Pour the sauce mixture over the chicken in the skillet. Stir everything to combine, ensuring the chicken is coated. Bring the sauce to a gentle simmer, then reduce the heat to low, cover the skillet, and let it cook for 20-25 minutes, or until the chicken is cooked through and tender enough to shred easily. The internal temperature of the chicken should reach 165°F (74°C).
6. Shred the Chicken: Remove the chicken breasts from the skillet and place them on a cutting board. Using two forks, shred the chicken into bite-sized pieces. It should pull apart with very little effort.
7. Finalize the Filling: Return the shredded chicken to the skillet with the sauce. Stir well to coat every piece of chicken. Let it simmer uncovered for another 5 minutes, allowing the sauce to thicken slightly and cling to the meat. Set the finished BBQ chicken filling aside. You can make this filling up to 3 days in advance to save time.

Part 2: Assembling and Grilling the Masterpiece

1. Set Up Your Station: Lay out your 8 slices of bread on a large cutting board or clean countertop. Have your softened butter, cheeses, shredded BBQ chicken filling, and thinly sliced red onion at the ready. A systematic assembly line is the key to efficiency.
2. Butter the Bread: Using a butter knife or offset spatula, spread a generous, even layer of softened butter on one side of each slice of bread. This buttered side will be the outside of your sandwich, responsible for creating that incredible golden crust.
3. Begin the Build: Place four of the bread slices butter-side down on your work surface. These are the foundations of your sandwiches.
4. Layer the Cheese: On each of the four bread slices, place one slice of sharp cheddar, followed by one slice of Monterey Jack (or your chosen melty cheese). If using shredded cheese, sprinkle a layer of each. Layering the cheese on the bottom creates a moisture barrier that helps prevent the bread from becoming soggy from the chicken filling.
5. Add the Star of the Show: Spoon a hearty amount of the warm BBQ chicken filling over the bottom layer of cheese. Spread it out evenly, but leave a small ½-inch border around the edges to prevent the filling from oozing out too much during grilling.
6. Add the Onion (Optional): Scatter a few thin slices of red onion over the chicken filling. This adds a wonderful, sharp crunch.
7. The Final Cheese Layer: Place another slice of Monterey Jack and another slice of sharp cheddar on top of the chicken. This top layer of cheese acts as a “glue,” holding the sandwich together as it melts.
8. Top it Off: Place the remaining four slices of bread on top, butter-side up. Gently press down on each sandwich to compact it slightly.
9. Grill to Golden Perfection: Heat a large non-stick skillet, cast-iron pan, or griddle over medium-low heat. The key to a perfect grilled cheese is “low and slow.” If the heat is too high, the bread will burn before the cheese has a chance to melt completely.
10. Cook the First Side: Carefully place one or two sandwiches in the preheated skillet (don’t overcrowd the pan). Cook for 4-6 minutes on the first side, until the bread is a deep golden brown and the bottom layer of cheese has begun to melt. You can press down gently with a spatula to encourage even cooking.
11. Flip and Finish: Using a wide spatula, carefully flip the sandwiches. Cook for another 4-6 minutes on the second side, until it’s equally golden brown and crispy, and the cheese is fully melted, gooey, and perhaps even oozing slightly from the sides.
12. Rest and Serve: Transfer the finished sandwiches to a cutting board and let them rest for a minute or two. This allows the molten cheese to set up slightly, making the sandwich easier to cut. Slice diagonally (it’s a scientific fact that they taste better this way!) and serve immediately while hot and melty.