Season the chicken thighs with salt and black pepper.
Place the chicken in the bottom of a slow cooker and top with diced onion.
In a small bowl, whisk together soy sauce, tomato paste, honey, sesame oil, sriracha, and grated ginger.
Pour the sauce over the chicken.
Cover and cook on low for 4 to 6 hours or on high for 2 to 3 hours, until chicken is cooked through and tender.
Remove the chicken and shred it using two forks.
In a small bowl, whisk together cornstarch and water to form a slurry.
Stir the slurry into the sauce in the crock pot and cook on high for 15–20 minutes, until the sauce thickens.
Return shredded chicken to the crock pot and stir to coat with sauce.
Sprinkle with sesame seeds and sliced scallions before serving.