Heat 1 tablespoon of the sesame oil in a skillet over medium heat.
Add the chicken pieces and sauté until cooked through, about 8–10 minutes.
Transfer the cooked chicken to the slow cooker.
In the same skillet, add the remaining 1 tablespoon sesame oil and sauté the diced onion until translucent, about 5 minutes; then add to the slow cooker.
Add the cooked brown rice, ground black pepper, frozen vegetable mix, and soy sauce to the slow cooker; stir to combine and scrape any browned bits from the skillet into the slow cooker.
Cook on low for 1 hour or on high for 30 minutes.
Meanwhile, scramble the large egg in a small pan until cooked and stir the scrambled egg into the crockpot just before serving.
Serve topped with sesame seeds and chopped green onion.
