Combine all of the Korean Barbecue Sauce ingredients listed above in a large mason jar. Screw on lid and vigorously shake until sauce ingredients are well combined and sauce is thickened.
Preheat oven to 350 degrees.
Heat oil and butter in a heavy oven-safe pot over medium-high heat note if you don’t have an oven safe pot that’s fine just use what you have and check note below before placing in the oven. Melt butter completely. Add chicken thighs, skin side down and cook until a light golden brown. Turn chicken over to brown the other side, cook until a light golden brown. Remove chicken from the pot and set aside on a plate.
Add diced onions to pan and cook for 3-5 minutes, stirring constantly. Pour in Korean barbecue sauce that you made prior. Return chicken to the pot, skin side down just to coat it, then immediately turn chicken over to skin side up. Cover pot and place it in the preheated oven
NOTE: if you don’t have an oven safe pot, simply transfer chicken and sauce to a 9x13 pan, cover with foil, and follow the remaining directions.
Cook for 45 minutes and then remove pot or pan from the oven. Remove lid and place back in oven under low broil for 5-6 minutes to give the sauce a nice caramelization, this is completely optional. Garnish chicken with chopped green onions.
Serve with fresh vegetables, couscous, rice or mashed potatoes.