Pat fully thawed thighs dry, trim large pieces of fat and arrange in one layer in a 4- to 6-quart slow cooker.
Whisk soy sauce, brown sugar, 2 tablespoons water, rice vinegar, sesame oil, ginger, garlic and pepper. Pour over chicken and turn once to coat.
Cover and cook on low 3 to 4 hours or high 1 1/2 to 2 1/2 hours, beginning checks at the earliest time.
Remove each thigh when its thickest center reaches 165°F. Rest on a clean plate for 5 minutes.
Strain cooking liquid into a saucepan. Bring to a full boil for at least 1 minute, stirring, and skim excess fat.
Whisk cornstarch with remaining 2 tablespoons cool water. Add slurry gradually to simmering sauce until it coats a spoon.
Slice rested chicken, coat with enough hot teriyaki sauce and garnish with scallions and toasted sesame seeds.
