Preheat the oven to 425°F (220°C). Line a baking sheet with foil or parchment and set aside.
In a bowl, whisk together 1/2 cup low sodium soy sauce, 1/4 cup rice vinegar, 1/4 cup brown sugar, 1 tablespoon grated ginger, 4 minced garlic cloves, 1 teaspoon toasted sesame oil, and 1/2 teaspoon sambal oelek until the sugar dissolves.
Reserve 1/4 cup of the sauce in a small bowl for basting, then pour the remaining sauce over the 1.5 pounds of chicken thighs in a shallow dish, turning to coat and submerge as much as possible.
Marinate the chicken for at least 30 minutes, or refrigerate overnight for more flavor.
When ready to cook, transfer the chicken to the prepared baking sheet, letting excess marinade drip off. Discard any remaining marinade from the bowl.
Bake the chicken at 425°F for 10 minutes.
Remove the baking sheet, brush the chicken with the reserved 1/4 cup sauce, then return to the oven and bake 5 more minutes.
Brush the chicken with sauce again, then bake an additional 3–5 minutes, or until an instant-read thermometer inserted into the thickest part reads 165°F; total baking time should be about 18–25 minutes.
When done, remove from the oven and sprinkle with sesame seeds and sliced scallions if desired, then serve.