In a medium bowl, whisk together soy sauce, water, sugar, garlic, and ginger until the sugar’s dissolved and you’re proud of yourself.
Place chicken thighs in a large Ziploc bag or a shallow dish. Pour in the marinade. Cover or seal and let it hang out in the fridge for at least 4 hours, or overnight if you're planning ahead (look at you!).
Preheat your grill or grill pan over medium heat. Remove chicken from marinade and discard the marinade like the strong, decisive cook you are.
Grill chicken for about 5-7 minutes on each side until nicely charred and fully cooked (165°F inside). Let it rest for a few minutes – it’s earned it.
Sprinkle with green onions and sesame seeds before serving, if you’re feeling fancy.