To make the sauce, add the ketchup, brown sugar, soy sauce, pineapple juice, garlic, ginger paste, rice vinegar, and sesame oil to a small saucepan. Heat over medium-low, stirring occasionally, and let it simmer for 15-20 minutes, or until the sauce has reduced by about half and is thick and glossy. Set aside.
Preheat your oven to 425°F and line a large baking sheet with parchment paper.
Cut the chicken thighs into bite-sized pieces and place them in a large bowl. Add 1 cup of the prepared BBQ sauce and toss until the chicken is evenly coated.
Spread the chicken onto the prepared baking sheet in an even layer. Add the pineapple, bell peppers, and onion, spreading everything out so it cooks evenly. Season with a light sprinkle of salt over everything.
Bake for 25-30 minutes, stirring halfway through, until the chicken is fully cooked, the vegetables are tender, and everything has started to caramelize around the edges.
Remove the sheet pan from the oven and brush everything with a little more of the BBQ sauce. Garnish with sliced green onions and sesame seeds, then serve over your favorite rice.