Start by peeling the ginger and garlic. Chop them finely and set aside.

In a large pot, combine shoyu, chicken broth, Worcestershire sauce, white vinegar, sugar, mirin, chopped ginger, and garlic. Place the pot over low heat and gently stir to allow the flavors to blend. If the sauce tastes too strong, add a small amount of water.

Lightly season the chicken thighs with salt and black pepper on both sides.

Heat a lightly oiled frying pan over medium heat. Sear the chicken thighs skin-side down until the skin becomes golden and some fat renders out. Flip and lightly sear the other side. Do not cook fully.

Transfer the seared chicken into the sauce pot. Bring the sauce to a boil, then reduce the heat to a gentle simmer. Cover and cook for 40 to 45 minutes, or up to 1 hour, until the chicken is fully cooked and tender.

Remove the chicken from the pot and set aside.

In a bowl, mix water first, then whisk in the flour to create a smooth slurry. Bring the sauce back to a rolling boil and slowly add the slurry while stirring. Reduce heat and simmer until the sauce thickens.

Add the chicken back into the pot, coat it well with the thickened sauce, and remove from heat. Serve hot over white rice and garnish with spring onions.