Heat oil in a large skillet in high heat. Add chopped onion and cook for 1-2 mins until charred while stirring. Add pinch of salt, garlic and ginger and cook additional 30 seconds. Set aside in a bowl.
Using the same skillet, add little bit of oil and cook chicken thighs, skin side down first. Cook for about 5-6 mins until golden brown, turning halfway. Use the lid to avoid splattering.
Meanwhile, make the sauce. Combine all the sauce ingredients and stir until well combined. Set aside.
Into the slow cooker, first layer the bottom with cooked onion, next chicken, then drizzle the sauce evenly all over. Close lid and set the slow cooker on high and cook for 1½ hour. Glaze meat occasionally.
When time it up, use a ladle to collect any visible floating fat liquid. Set it aside.
Add in slurry, red pepper and give it a stir. Close lid and cook additional 20 mins.
Turn off slow cooker and stir in pineapples. Garnish with scallion and sesame seeds. Serve with hot rice.