1. Heat the oil in a medium skillet over a medium-high heat. Add the garlic and ginger and cook until fragrant, about 1 minute. Add the diced onion and ½ teaspoon of salt, reduce the heat to medium-low, and cook gently until the onions are soft and beginning to brown.
2. Add the tomato paste, water, paprika, cumin and turmeric, and cook for 5 minutes over medium heat.
3. Thinly sliver the clementine peel and add it to the pan, along with the slit chiles and ½ teaspoon of salt. Cook for about 10 minutes—until the peel is soft and almost falling apart—then roughly mash it, avoiding the chiles.
4. Add the chunks of fish. Cover and cook over low heat until the fish is just cooked through, 6 to 8 minutes.
5. Take the pan off the heat. Squeeze in the clementine juice, taste the sauce, and adjust seasonings. Sprinkle with cilantro, and serve with hot basmati rice or some warm flatbread.