Dice the onion into 1/2-inch pieces, thinly slice the bell pepper, mince the garlic, and chop the pineapple into 1/2-inch chunks. Pat the chicken breasts dry with paper towels and season both sides generously with salt and pepper—this helps the chicken develop better flavor during the long cook. Set all prepped ingredients within arm's reach of your slow cooker.

In a bowl, whisk together the ketchup, pineapple juice, soy sauce, honey, vinegar, ginger, onion powder, garlic powder, nutmeg, and sriracha until smooth and well combined. This creates a balanced sweet-savory-spicy flavor base. Add the diced onion and sliced bell pepper to the slow cooker, then nestle the chicken breasts on top. Pour the sauce mixture over the chicken and aromatics, then scatter the minced garlic over everything.

Cover the slow cooker and cook on low for 4 to 6 hours, until the chicken is fall-apart tender and easily shreds with a fork. I like to check it around the 4-hour mark—this ensures you don't overcook the chicken, which can become stringy. The longer cooking time allows the spices to fully infuse into the meat and vegetables while the sauce builds deeper flavor.

Transfer the cooked chicken to a plate and let it cool for a few minutes, then shred it using two forks or your hands. Return the shredded chicken to the slow cooker with the sauce and vegetables. In a small bowl, whisk the cornstarch with 2 tablespoons of water until smooth, then stir this slurry into the slow cooker. Cook on low for an additional 5 minutes, stirring occasionally, until the sauce thickens slightly and coats the chicken nicely.

Fold the chopped pineapple chunks into the shredded chicken mixture and cook on low for 15 to 20 minutes to warm through and allow the pineapple to infuse into the sauce. Taste the mixture and adjust seasoning if needed—add a squeeze of lime juice for brightness or a dash more sriracha for heat. I find the pineapple adds sweetness and texture at the end, so it's best added toward the finish to maintain its character rather than letting it break down completely.

While the pineapple finishes cooking, split the brioche rolls and lightly butter the insides. Toast them in a skillet over medium heat or under the broiler for 1 to 2 minutes until golden and crispy on the inside—this prevents the rolls from getting soggy and adds a pleasant textural contrast. Generously spoon the Hawaiian chicken mixture onto each toasted roll, top with fresh green onions, and serve immediately with lime wedges on the side for squeezing.