Pat the 8 boneless chicken thighs dry and evenly rub with 1 tablespoon Chinese 5-spice powder.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. When hot, add the chicken and sear until browned, about 3–4 minutes per side. If the skillet is crowded, sear in batches. Transfer seared chicken to a plate.
Place the 2 white onions (roughly chopped) in the bottom of the slow cooker, then arrange the seared chicken on top of the onions.
In a bowl, whisk together 1 cup low-sodium chicken broth, 1/3 cup rice vinegar, 1/3 cup tamari, 1 tablespoon coconut sugar, 1 teaspoon red chili flakes, and 1 bay leaf. Pour the mixture evenly over the chicken and onions in the slow cooker.
Cover and cook on low for 6–8 hours or on high for 3–4 hours, until the chicken is cooked through and tender.
About 10 minutes before the end of the cooking time, add 1/2 cup pea pods to the slow cooker, cover, and cook until the pods are tender (about 10 minutes).
Remove and discard the bay leaf. Serve the chicken topped with the 2 sliced scallions (white and green parts).