Spray the inside of your slow cooker bowl with nonstick cooking spray. Stir together gochujang, soy sauce, onion, garlic, ginger, mirin, brown sugar, sesame oil, and pepper in slow cooker bowl until fully combined. Add chicken and stir to evenly coat.
Cover slow cooker and cook on low for about 4 hours, until the chicken is fully cooked. Remove chicken to a large bowl. Using 2 forks, shred chicken. Meanwhile, fully combine arrowroot powder/cornstarch with water in a small bowl. Pour into the sauce in slow cooker, stirring to incorporate. Add chicken back into slow cooker and stir into sauce. Cook for 10-15 more minutes.
Place a bit of chicken on each bun or lettuce wrap, topping with kimchi, avocado, lightly pickled bean sprouts, gochujang aioli, scallions, and sesame seeds. Devour immediately.
for the Lightly Pickled Bean Sprouts
Combine vinegar, warm water, sugar, and salt in a small bowl until dissolved. Pour over bean sprouts and let sit at room temperature for 30 minutes.
for the Gochujang Aioli
Combine all ingredients in a small bowl until fully incorporated. Can be made ahead and refrigerated.