Start by cutting the chicken breasts into 4 even pieces, then pound each to a uniform 1/2-inch thickness—this ensures they'll cook evenly on the grill. While the chicken is still on your work surface, season both sides generously with salt, pepper, garlic powder, and smoked paprika, pressing the spices in gently so they adhere well. In a separate bowl, whisk together the pineapple juice, soy sauce, brown sugar, and mustard until the sugar dissolves completely. Place the seasoned chicken in a zip-top bag or shallow dish, pour the marinade over it, and refrigerate for at least 2 hours (or overnight for deeper flavor). I find that even a 2-hour marinade develops plenty of flavor if you're short on time, though overnight is ideal.

While the chicken marinates, pat the pineapple rings completely dry with paper towels—this is crucial for achieving a nice caramelized crust rather than steaming them on the grill. Slice the red onions into thin slices and set aside. Spread the mayonnaise evenly on the inside of all 4 buns and set them on a clean surface, ready for assembly. Have the provolone slices ready as well. This prep work takes just a few minutes but makes the final assembly seamless once everything is hot off the grill.

Preheat your grill to medium-high heat (around 400°F if using a gas grill). Remove the chicken from the marinade and let any excess drip off, then place the pieces directly on the grill grates. Cook for about 6-7 minutes without moving them, which allows the spices to caramelize and creates beautiful grill marks. After about 6 minutes, add the pineapple rings to a cooler part of the grill or on the outer edge—they'll need about 6-8 minutes total, getting flipped once halfway through. When the chicken has cooked for about 12 minutes total (6 minutes per side), it should register 165°F on a meat thermometer. I like to flip the chicken only once to keep those grill marks intact and let the pineapple caramelize undisturbed.

During the final 2-3 minutes of grilling the chicken, place one slice of provolone on top of each chicken cutlet. If your grill has a lid, close it to help the cheese melt faster—it should become soft and slightly bubbly. If you don't have a lid, you can tent the chicken loosely with foil. The cheese will continue to melt off the grill if needed, but getting it warm and pliable while still grilling is ideal.

Remove the chicken and pineapple from the grill and let the chicken rest for just 1-2 minutes—this keeps the juices from running out when you bite into it. Place one piece of grilled chicken with melted cheese on each mayo-spread bun, then top with a grilled pineapple ring. Add a few slices of fresh red onion on top of the pineapple for a pop of color and a sharp, fresh contrast to the sweet and savory flavors. Serve immediately while everything is still warm.