Step 1: Prepare the Slow Cooker

1. Spray the slow cooker insert with nonstick cooking spray or lightly coat it with oil to prevent sticking.
2. Place the sliced onion, minced garlic, and grated ginger in the bottom of the slow cooker. These aromatics will infuse the dish with flavor as it cooks.

Step 2: Season and Add the Chicken

1. Season the chicken thighs (or breasts) with salt and black pepper on both sides.
2. Arrange the chicken pieces in a single layer over the onion, garlic, and ginger in the slow cooker.

Step 3: Make the Peanut Sauce

1. In a medium mixing bowl, whisk together the peanut butter, coconut milk, soy sauce, rice vinegar, brown sugar, red curry paste, and chili flakes (if using). Whisk until the mixture is smooth and well combined.
2. Taste the sauce and adjust the seasoning as needed. Add a bit more soy sauce for saltiness, brown sugar for sweetness, or curry paste for spiciness.

Step 4: Pour the Sauce Over the Chicken

1. Pour the peanut sauce evenly over the chicken, ensuring all pieces are coated.
2. Cover the slow cooker with the lid and set it to cook on low for 4-6 hours or high for 2-3 hours.

Step 5: Shred or Slice the Chicken (Optional)

1. Once the chicken is fully cooked (internal temperature should reach 165°F/74°C), remove it from the slow cooker and place it on a cutting board.
2. If desired, shred the chicken using two forks or slice it into thin strips. Return the chicken to the slow cooker and stir it into the sauce.

Step 6: Garnish and Serve

1. Serve the peanut chicken over steamed rice, noodles, or cauliflower rice.
2. Garnish with fresh cilantro, crushed peanuts, lime wedges, and sliced green onions for added flavor and texture.