Pat the chicken pieces completely dry with paper towels and season generously with salt and pepper. Heat the olive oil in a large Dutch oven or skillet over medium-high heat. Once shimmering, add the chicken in a single layer and sear for 3-4 minutes per side until deeply golden brown. Remove the chicken to a plate and set aside.
Reduce the heat to medium. Add the diced onion to the pot and sauté for 4-5 minutes, scraping up any browned bits from the bottom, until softened and translucent. Add the minced garlic and cook for 1 more minute until fragrant.
Pour in the chicken broth, the reserved pineapple juice, soy sauce, and rice vinegar. Bring the liquid to a simmer, using a wooden spoon to scrape the bottom of the pot to deglaze all the flavorful bits.
Stir in the rinsed rice, ensuring it is fully submerged. Return the seared chicken to the pot, nestling it into the rice and liquid. Bring the mixture back to a gentle simmer.
Once simmering, reduce the heat to low, cover the pot with a tight-fitting lid, and cook for 18-20 minutes without peeking. The steam is crucial for cooking the rice perfectly.
In a small bowl, whisk the cornstarch and cold water together to create a slurry. Uncover the pot, stir in the slurry and the pineapple chunks. Increase the heat to medium and allow it to bubble for 1-2 minutes, stirring gently, until the sauce has thickened slightly.
Remove the pot from the heat, cover it again, and let it rest for 5-10 minutes. This allows the rice to finish steaming. Before serving, fluff the rice gently with a fork and garnish with sliced green onions, toasted sesame seeds, and red pepper flakes, if desired.