If you prefer, you can sear the chicken thighs in a skillet with a small amount of oil for extra flavor before adding them to the slow cooker. This step is optional but recommended for a deeper color and taste.
In a bowl, whisk together peanut butter, coconut milk, chicken broth, soy sauce, fish sauce (if using), lime juice, brown sugar, grated ginger, minced garlic, red curry paste, and red pepper flakes until smooth.
Place the chicken thighs in the slow cooker. Pour the prepared peanut sauce over the chicken, making sure the pieces are well coated.
Cover and cook on low for 6–8 hours or on high for 3–4 hours, until the chicken is tender and easily shreddable.
If you like a thicker sauce, remove the lid in the last 15–20 minutes of cooking and turn the slow cooker to high to reduce slightly, or whisk in a teaspoon of cornstarch slurry (1 teaspoon cornstarch mixed with 1 teaspoon water) and cook for a few more minutes.
Shred or slice the chicken directly in the sauce, then taste and adjust seasoning with more lime juice, soy sauce, or a pinch of sugar if desired.
Serve over hot cooked rice or noodles. Garnish with chopped cilantro and lime wedges for a bright finish.