Add all of the ingredients, except the chicken, into the slow cooker. Stir to combine.
1 small onion, 4 garlic cloves, 1 inch fresh ginger, 2 red bell peppers, 2 Thai red chillies, 1 stem lemongrass, zest and juice of 1 lime, 1 can full fat coconut milk, ¼ cup Thai red curry paste, 1 cup peanut butter, 1 tablespoon fish sauce