In a large skillet over medium heat, heat sesame oil until shimmering. Season the diced chicken with salt and pepper, then add it to the skillet, cooking for about 6-7 minutes, or until it is nicely browned and cooked through. Remove the chicken and set it aside.
In the same skillet, add minced garlic and grated ginger. Sauté for about 30 seconds until fragrant, making sure not to burn.
Stir in the soy sauce, pineapple juice, and cornstarch. Mix well to create a smooth glaze, and bring it to a gentle simmer until it thickens, which should take about 2-3 minutes.
Add the chicken back to the skillet along with the cooked rice, tossing everything together to combine. Gently fold in the pineapple chunks and sprinkle with red pepper flakes if using. Allow it to simmer for another 3-4 minutes, letting all the flavors come together.
Once everything is heated through, serve the pineapple chicken over a bed of rice, drizzling any extra sauce on top. Enjoy your meal!