Whisk the soy sauce, pineapple juice, brown sugar, ginger, garlic, and sesame oil together in a bowl. Reserve half of the mixture in a small saucepan.
Marinate the chicken thighs in the remaining liquid for at least 30 minutes in a shallow dish.
Simmer the reserved sauce over medium heat until it reduces by half and becomes glossy.
Grill the marinated chicken in a skillet over medium-high heat for 6 minutes per side. Add the pineapple rings and onion slices during the last 3 minutes to char lightly.
Assemble the stacks by layering compacted rice, an onion ring, a chicken thigh, and a pineapple ring. Drizzle with the reduced glaze to serve.
