Season the chicken thighs lightly with salt and pepper.
Heat 1 tablespoon olive oil in a large skillet over high heat and sear the chicken 1–2 minutes per side until browned but not cooked through; this step is optional. Transfer the seared chicken to the crockpot.
In a mixing bowl, combine the canned pineapple chunks with their juice (reserve 3 tablespoons of juice), brown sugar, vinegar, diced bell pepper, diced onion, pressed garlic, ginger, and ketchup. Stir until combined.
Pour the pineapple mixture over the chicken in the crockpot, spreading the pieces and sauce evenly.
Cook on low for 7–8 hours or on high for 3–4 hours, until the chicken is tender and cooked through.
About 30 minutes before the end of cooking, whisk the reserved 3 tablespoons pineapple juice with 2 tablespoons cornstarch until smooth, then gently stir the slurry into the crockpot to thicken the sauce. Continue cooking for the remaining time.
When done, transfer chicken to a serving dish or shred in the crockpot if desired. Top with finely sliced green onions and serve over rice.
