Rinse rice until water runs clear. Combine with water and salt in a saucepan. Bring to a boil, reduce to lowest heat, cover and cook 15 minutes. Remove from heat and steam covered 5 minutes. Fluff with fork.
Combine soy sauce, mirin, sugar, water, ginger, and garlic in a small saucepan over medium heat. Stir until sugar dissolves and bring to a gentle simmer.
Add cornstarch slurry. Stir constantly for 1–2 minutes until sauce is glossy and coats the back of a spoon. Remove from heat.
Pat chicken thighs completely dry. Season both sides with salt and pepper.
Heat oil in a large skillet over medium-high until shimmering. Add chicken smooth side down. Do not move.
Cook 5–6 minutes until deep golden and releasing naturally from the pan.
Flip and cook 4–5 minutes until cooked through to 74°C (165°F).
Reduce heat to medium-low. Pour half the teriyaki sauce over the chicken. Toss and cook 1–2 minutes until glazed and caramelized at the edges.
Transfer to cutting board. Rest 3–4 minutes. Slice into strips.
Divide rice between four bowls. Lay sliced chicken over the rice. Spoon remaining teriyaki sauce over the chicken.
Arrange cucumber, carrot, and avocado around the chicken.
Scatter spring onions, sesame seeds, and nori strips over the top.
Serve with lime wedges, sriracha, and extra soy sauce on the side.