In a slow cooker, place the chicken pieces, diced onion, and red bell pepper; season with salt and pepper, tossing to coat.
To make the peanut sauce, in a bowl, combine the peanut sauce ingredients (garlic through red pepper flakes). Whisk to combine.
Pour the sauce over chicken and veggies in slow cooker. Cook on low heat for 4 to 4 ½ hours or until chicken is cooked through.
Taste and season with salt and pepper as desired.
Remove lid and stir in the spinach leaves and lime juice. Stir and heat a few more minutes to allow the spinach to wilt into the sauce.
If desired, serve over rice, cauliflower rice, pad Thai noodles, or any noodles. Top with desired garnishes.
