Mix miso paste, soy sauce, vinegar, sesame and grapeseed oil, ginger and garlic in a measuring cup or small bowl and whisk together until completely combined.
Trim excess fat from chicken thighs, put on a plate and brush generously with the miso glaze on both sides. Reserve leftover glaze. Season chicken on the skin side with black pepper, if using (we like the appearance as well as the flavor it adds).
Let chicken marinate in the glaze for at least 30 minutes at room temperature, or up to overnight (transfer to a lidded container and refrigerate for longer marinating, then take it out of the fridge about 30 minutes before roasting to warm up).
Preheat the oven to 400ºF, with a rack in the middle position. Prepare a hotel pan or shallow roasting pan—either use a silicone baking mat or coat the pan with oil to reduce sticking. Arrange chicken in a single layer, skin side up.
Roast chicken for 30 to 40 minutes, basting the chicken with reserved glaze at 15 minutes. Check at the 30-minute mark; an instant read thermometer should read at least 160ºF when inserted in the thickest part. Transfer chicken to platter, top with scallions and serve.