Prep the slow cooker base: Lightly grease the slow cooker insert.
Layer sliced bell pepper and onion on the bottom if using.
Make the sauce: In a bowl, whisk coconut milk, peanut butter, soy sauce (or aminos), lime juice, rice vinegar, sambal or sriracha, garlic, ginger, sesame oil, sweetener (if using), and chicken broth until smooth.
Add the chicken: Place chicken thighs in the slow cooker. Pour the sauce over the top, making sure the chicken is mostly submerged.
Cook low and slow: Cover and cook on Low for 4–5 hours or on High for 2.5–3 hours, until the chicken is tender and easily shredded.
Shred and thicken (optional): Remove chicken to a plate and shred with two forks. If you prefer a thicker sauce, sprinkle in xanthan gum a tiny pinch at a time while whisking.
Return shredded chicken to the slow cooker and stir to coat.
Taste and adjust: Add more lime for brightness, soy for salt, or chili for heat. The sauce should be creamy, savory, slightly tangy, and balanced.
Serve: Spoon over cauliflower rice, zucchini noodles, or steamed broccoli. Top with chopped cilantro, crushed peanuts, green onions, and a squeeze of lime.