1. Prepare Chicken in Slow Cooker with Initial Sauce:
- Place the boneless, skinless chicken breasts into a 4-quart slow cooker.
- In a small bowl, combine 1/4 cup of the sweet and spicy barbecue sauce with the 2 tablespoons of honey mustard. Stir well.
- Pour this initial sauce mixture evenly over the chicken breasts in the slow cooker.
2. Slow Cook the Chicken:
- Cover the slow cooker securely with its lid.
- Cook on the LOW setting for 2 1/2 to 3 hours. The chicken is done when it is very tender and an instant-read thermometer inserted into the thickest part reads 165°F (74°C). (Note: 2.5-3 hours on LOW for chicken breasts is quite fast; ensure they are truly tender enough to shred. If using larger/thicker breasts, or if your slow cooker runs cooler, it might take longer, up to 4-5 hours on LOW).
3. Shred Chicken and Add Remaining Sauce & Pineapple:
- Once the chicken is tender, carefully remove the chicken breasts from the slow cooker and place them on a clean cutting board or large plate. Leave the cooking liquid that has accumulated in the slow cooker for now.
- Using two forks, shred the cooked chicken.
- Discard the cooking liquid from the slow cooker (it may be watery and too diluted).
- Return the shredded chicken to the now-empty slow cooker.
- Add the undrained can of crushed pineapple and the remaining barbecue sauce (the rest of the 18-ounce bottle) to the shredded chicken in the slow cooker.
- Stir everything together well until the shredded chicken and pineapple are evenly coated with the BBQ sauce.
4. Heat Through:
- Cover the slow cooker again. Cook on the HIGH setting for about 15 minutes, or just until the entire mixture is heated through and the flavors have melded.
5. Serve:
- While the chicken mixture heats, split and toast your hamburger buns to your liking.
- Spoon the hot Sweet & Spicy Pineapple Chicken mixture generously onto the toasted buns.
- If desired, top with Bibb lettuce leaves and/or thinly sliced red onion before adding the top bun.
- Serve the sandwiches immediately while warm. Enjoy!