Pat the chicken dry and season lightly with salt and pepper. For extra flavor, sear the chicken 1–2 minutes per side in a hot skillet, then transfer to the slow cooker.
In a bowl, whisk peanut butter, coconut milk, soy sauce, brown sugar, rice vinegar (or lime), garlic, ginger, sriracha, chicken broth, and fish sauce until smooth.
Place the sliced onion in the bottom of the crockpot, add the chicken, and pour the sauce over. Cover and cook on LOW for 4–5 hours or HIGH for 2–3 hours, until the chicken shreds easily.
Remove the chicken, shred with two forks, and return it to the sauce. Stir thoroughly so every strand is coated.
If desired, stir in the cornstarch slurry and cook on HIGH for 10–15 minutes until glossy and thickened. Adjust seasoning with lime juice, sriracha, or sugar as needed. Garnish and serve.
Notes
Make-ahead: Sauce can be mixed the day before. Store separately in the fridge and add to the crockpot when ready to cook.
Freezing: Freeze in portions for up to 3 months. Thaw overnight in the fridge before reheating.
Low-carb option: Serve over cauliflower rice or inside lettuce cups instead of grain-based sides.