Bring water to boil in the rice cooker.
Remove bone from chicken thighs. Reserve the bones. Place on a plate with flesh side up. Sprinkle with salt and pepper.
Cut two green onions in half crosswise. Then split them in half lengthwise. Cut remaining green onions thinly for garnishing.
Arrange half of the green onions and ginger in rice cooker steaming rack. Transfer chicken into the steaming rack skin side up on top of the green onions and ginger. Arrange the remaining green onions and ginger on top of the chicken. Cover with lid.
Place the steaming rack onto the rice cooker and steam chicken for 7 minutes.
In the meantime, prepare the sauce and rice. Heat vegetable oil in a small sauce pan. Saute garlic for 30 seconds. Pour half the garlic oil into a small bowl.
Return saucepan with remaining garlic oil to the stove. Add rice and stir to coat the rice with the garlic oil. Turn off stove.
After 7 minutes, remove the steaming rack from the rice cooker. Scoop out 1½ cups (360ml) chicken broth from the rice cooker leaving ¾ cup (180ml) in the pot. Transfer the rice from the saucepan into the rice cooker pot. Stir in salt to taste. Return the steaming rack on top of the rice cooker to continue cooking.
Add soy sauce, oyster sauce, and sesame oil to the small bowl with garlic oil. Mix well with a spoon. Set aside.
Pour reserved chicken broth into a small saucepan. Add reserve chicken thigh bones to the saucepan. Cover and bring chicken broth to a boil. Reduce heat and simmer for 10 minutes.
After 10 minutes, remove the bones from the saucepan. Season with salt and pepper to taste. Add Napa cabbage. Cover and boil for 2 minutes. Turn off stove.
By now, rice and chicken should be cooked and rice cooker should be in keep warm mode. Turn off the rice cooker. Scoop up a bowl of rice onto a plate. Place a few slices of cucumber on the side.
Cut chicken into ½ inch pieces and place them on the cucumber slices. Drizzle garlic oyster sauce on the chicken.
Serve immediately with garlic chili sauce and a bowl of soup.