Chop the onion, mince the garlic and ginger and peel and dice the sweet potato.
In a large pot, heat coconut oil and cook the onion for 2-3 minutes over medium heat. Add the garlic, ginger, chopped chilli pepper (if using) and the spices and stir around for a 2 minutes until fragrant, without burning.
Add the sweet potato and pour over the broth. Cook uncovered for 10 minutes.
Cut the fish fillets into medium-sized pieces and season with a little salt and pepper. Add to the stew, along with the coconut milk, and cook for 10 minutes. Wash the spinach leaves and stir them in towards the end. Turn off heat, cover, and let sit for 5 minutes before serving with fresh cilantro and lemon wedges.