In a medium bowl, whisk together the pineapple juice, soy sauce, ketchup, brown sugar, and tomato paste until the sugar dissolves completely. Add the sesame oil, minced garlic, grated ginger, red pepper flakes, nutmeg, chili powder, rice vinegar, and kosher salt. Stir well to combine all aromatics and spices evenly. I like to taste the marinade at this point—it should have a balance of sweet, salty, and umami with a subtle heat and warm spice from the nutmeg. Divide the marinade into two equal portions: one for marinating the chicken and one for basting during cooking.
Pat the chicken thighs dry with paper towels and place them in a large bowl or resealable bag. Pour half of the marinade over the chicken, ensuring each piece is well coated. Let the chicken marinate at room temperature for 20 minutes—this brief marinating window is enough to infuse flavor without over-softening the meat. While the chicken marinates, prepare your cooking vessel: heat your grill to medium-high heat (for grilling), or preheat your oven to 375°F or air fryer to 350°F if using those methods. Set the remaining marinade aside in a small saucepan.
For grilling: Remove chicken from marinade and grill 8-10 minutes per side over medium-high heat, brushing with the reserved marinade halfway through cooking. For oven: Place marinated chicken on a lined baking sheet and bake at 375°F for 16-18 minutes, brushing with reserved marinade halfway through. For air fryer: Arrange chicken in the basket and cook at 350°F for 15-18 minutes, brushing with reserved marinade at the 7-8 minute mark. In all methods, the chicken is done when the internal temperature reaches 165°F at the thickest part. I always brush the sauce on during the last half of cooking—this prevents it from burning while still caramelizing the chicken beautifully.
While the chicken is finishing its final few minutes of cooking, grill the pineapple rings on the same heat source (or in the oven/air fryer with the chicken) for about 4 minutes, just long enough to develop light caramelization and grill marks. If grilling, place rings directly on the grates; if using the oven or air fryer, place them on the same sheet or basket with the chicken. The pineapple should be warm and slightly softened but still hold its shape.
Remove the cooked chicken from heat and let it rest for 5 minutes—this allows the juices to redistribute, keeping the meat moist and tender. While the chicken rests, prepare the serving plates with warm cooked rice as the base. Arrange the rested chicken thighs on the rice and place a grilled pineapple ring on or next to each piece. Scatter the cubed avocado over the plate and finish with a generous sprinkle of fresh cilantro and thinly sliced green onions. The combination of warm chicken, cool creamy avocado, and bright fresh herbs creates the perfect balance of flavors.