Add the onion, ginger, garlic, chicken thighs, peanut butter, coconut milk, crumbled chicken stock cube, soy sauce, curry powder, and brown sugar to the slow cooker. Stir everything together until well combined.
Cover and cook on low for 6–8 hours or on high for 4 hours, until the chicken is cooked through and tender.
About 1 hour before the end of cooking time, add the chopped pumpkin and zucchini to the slow cooker. Stir gently to incorporate.
Once cooking is complete, stir through the baby spinach leaves until wilted.
