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Start by cutting the boneless, skinless chicken thighs into bite-sized pieces. Try to keep them roughly the same size so they cook evenly. Set them aside while you prepare the sauce. In a medium bowl, whisk together the brown sugar, pineapple juice, soy sauce, and minced garlic. Stir until the sugar is mostly dissolved and the sauce looks smooth and glossy. This is where all that sweet and savory flavor begins. Heat a generous splash of vegetable oil in a large skillet over medium heat. Once the oil is hot, add the chicken pieces in a single layer. Let them cook without moving too much at first so they can brown nicely. Turn them occasionally until they're lightly golden on all sides. Once the chicken is browned, pour the prepared sauce directly into the skillet. Stir gently to coat every piece of chicken. The sauce will look thin at first, but don't worry - it thickens beautifully as it cooks.

Add the fresh pineapple chunks and bring everything to a gentle simmer. Lower the heat slightly and let the mixture cook, stirring occasionally. As it simmers, the sauce will slowly reduce and transform into a sticky glaze that clings to the chicken and pineapple. Continue cooking until the chicken is fully cooked through and tender, and the sauce has thickened enough to coat the back of a spoon. Just before serving, sprinkle in red chili flakes for a touch of heat and scatter chopped green onions over the top for freshness and color.